# Re: \[PATCH\] epoll: add exclusive wakeups flag - List: linux-api - 8 messages, 3 authors - Page: https://ratatoskr.run/linux-api/2016/03/105829/t ## Re: \[PATCH\] epoll: add exclusive wakeups flag - From: Michael Kerrisk (man-pages) \ - Date: 2016-03-14 17:47:45 - Message: https://ratatoskr.run/linux-api/2016/03/105829.md ```text [Restoring CC, which I see I accidentally dropped, one iteration back.] Hi Jason, Thanks for the review. I've tweaked one piece to respond to your feedback. But I also have another new question below. On 03/15/2016 03:55 AM, Jason Baron wrote: > On 03/11/2016 06:25 PM, Michael Kerrisk (man-pages) wrote: >> On 03/11/2016 09:51 PM, Jason Baron wrote: >>> On 03/11/2016 03:30 PM, Michael Kerrisk (man-pages) wrote: [...] > Hi Michael, > > Looks good. One comment below. > > Thanks, > >> EPOLLEXCLUSIVE (since Linux 4.5) >> Sets an exclusive wakeup mode for the epoll file >> descriptor that is being attached to the target file >> descriptor, fd. When a wakeup event occurs and multiple >> epoll file descriptors are attached to the same target >> file using EPOLLEXCLUSIVE, one or more of the epoll file >> descriptors will receive an event with epoll_wait(2). >> The default in this scenario (when EPOLLEXCLUSIVE is not >> set) is for all epoll file descriptors to receive an >> event. EPOLLEXCLUSIVE is thus useful for avoiding thun‐ >> dering herd problems in certain scenarios. >> >> If the same file descriptor is in multiple epoll >> instances, some with the EPOLLEXCLUSIVE flag, and others >> without, then events will provided to all epoll >> instances that did not specify EPOLLEXCLUSIVE, and at >> least one of the epoll instances that did specify >> EPOLLEXCLUSIVE. >> >> The following values may be specified in conjunction >> with EPOLLEXCLUSIVE: EPOLLIN, EPOLLOUT, EPOLLWAKEUP, and >> EPOLLET. EPOLLHUP and EPOLLERR can also be specified, >> but are ignored (as usual). Attempts to specify other > > I'm not sure 'ignored' is the right wording here. 'EPOLLHUP' and > 'EPOLERR' are always included in the set of events when something is > added as EPOLLEXCLUSIVE. This is consistent with the non-EPOLLEXCLUSIVE > add case. Yes. > So 'EPOLLHUP' and 'EPOLERR' may be specified but will be > included in the set of events on an add, whether they are specified or not. Yes. I understand your discomfort with the work "ignored", but the problem was that, because it made special mention of EPOLLHUP and EPOLLERR, your proposed text made it sound as though EPOLLEXCLUSIVE somehow was special with respect to these two flags. I wanted to clarify that it is not. How about this: The following values may be specified in conjunction with EPOLLEXCLUSIVE: EPOLLIN, EPOLLOUT, EPOLLWAKEUP, and EPOLLET. EPOLLHUP and EPOLLERR can also be specified, but this is not required: as usual, these events are always reported if they occur, regardless of whether they are specified in events. ? >> values in events yield an error. EPOLLEXCLUSIVE may be >> used only in an EPOLL_CTL_ADD operation; attempts to >> employ it with EPOLL_CTL_MOD yield an error. If >> EPOLLEXCLUSIVE has set using epoll_ctl(2), then a subse‐ >> quent EPOLL_CTL_MOD on the same epfd, fd pair yields an b>> error. An epoll_ctl(2) that specifies EPOLLEXCLUSIVE in >> events and specifies the target file descriptor fd as an >> epoll instance will likewise fail. The error in all of >> these cases is EINVAL. >> >> ERRORS >> EINVAL An invalid event type was specified along with EPOLLEX‐ >> CLUSIVE in events. >> >> EINVAL op was EPOLL_CTL_MOD and events included EPOLLEXCLUSIVE. >> >> EINVAL op was EPOLL_CTL_MOD and the EPOLLEXCLUSIVE flag has >> previously been applied to this epfd, fd pair. >> >> EINVAL EPOLLEXCLUSIVE was specified in event and fd is refers >> to an epoll instance. Returning to the second sentence in this description: When a wakeup event occurs and multiple epoll file descrip‐ tors are attached to the same target file using EPOLLEXCLU‐ SIVE, one or more of the epoll file descriptors will receive an event with epoll_wait(2). There is a point that is unclear to me: what does "target file" refer to? Is it an open file description (aka open file table entry) or an inode? I suspect the former, but it was not clear in your original text. To make this point even clearer, here are two scenarios I'm thinking of. In each case, we're talking of monitoring the read end of a FIFO. === Scenario 1: We have three processes each of which 1. Creates an epoll instance 2. Opens the read end of the FIFO 3. Adds the read end of the FIFO to the epoll instance, specifying EPOLLEXCLUSIVE When input becomes available on the FIFO, how many processes get a wakeup? === Scenario 3 A parent process opens the read end of a FIFO and then calls fork() three times to create three children. Each child then: 1. Creates an epoll instance 2. Adds the read end of the FIFO to the epoll instance, specifying EPOLLEXCLUSIVE When input becomes available on the FIFO, how many processes get a wakeup? === Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ ``` ## Re: \[PATCH\] epoll: add exclusive wakeups flag - From: Jason Baron \ - Date: 2016-03-14 19:32:19 - Message: https://ratatoskr.run/linux-api/2016/03/105828.md ```text On 03/14/2016 01:47 PM, Michael Kerrisk (man-pages) wrote: > [Restoring CC, which I see I accidentally dropped, one iteration back.] > > Hi Jason, > > Thanks for the review. I've tweaked one piece to respond to your > feedback. But I also have another new question below. > > On 03/15/2016 03:55 AM, Jason Baron wrote: >> On 03/11/2016 06:25 PM, Michael Kerrisk (man-pages) wrote: >>> On 03/11/2016 09:51 PM, Jason Baron wrote: >>>> On 03/11/2016 03:30 PM, Michael Kerrisk (man-pages) wrote: > > [...] > >> Hi Michael, >> >> Looks good. One comment below. >> >> Thanks, >> >>> EPOLLEXCLUSIVE (since Linux 4.5) >>> Sets an exclusive wakeup mode for the epoll file >>> descriptor that is being attached to the target file >>> descriptor, fd. When a wakeup event occurs and multiple >>> epoll file descriptors are attached to the same target >>> file using EPOLLEXCLUSIVE, one or more of the epoll file >>> descriptors will receive an event with epoll_wait(2). >>> The default in this scenario (when EPOLLEXCLUSIVE is not >>> set) is for all epoll file descriptors to receive an >>> event. EPOLLEXCLUSIVE is thus useful for avoiding thun‐ >>> dering herd problems in certain scenarios. >>> >>> If the same file descriptor is in multiple epoll >>> instances, some with the EPOLLEXCLUSIVE flag, and others >>> without, then events will provided to all epoll >>> instances that did not specify EPOLLEXCLUSIVE, and at >>> least one of the epoll instances that did specify >>> EPOLLEXCLUSIVE. >>> >>> The following values may be specified in conjunction >>> with EPOLLEXCLUSIVE: EPOLLIN, EPOLLOUT, EPOLLWAKEUP, and >>> EPOLLET. EPOLLHUP and EPOLLERR can also be specified, >>> but are ignored (as usual). Attempts to specify other >> >> I'm not sure 'ignored' is the right wording here. 'EPOLLHUP' and >> 'EPOLERR' are always included in the set of events when something is >> added as EPOLLEXCLUSIVE. This is consistent with the non-EPOLLEXCLUSIVE >> add case. > > Yes. > >> So 'EPOLLHUP' and 'EPOLERR' may be specified but will be >> included in the set of events on an add, whether they are specified or not. > > Yes. I understand your discomfort with the work "ignored", but the > problem was that, because it made special mention of EPOLLHUP and EPOLLERR, > your proposed text made it sound as though EPOLLEXCLUSIVE somehow was > special with respect to these two flags. I wanted to clarify that it is not. > How about this: > > The following values may be specified in conjunction > with EPOLLEXCLUSIVE: EPOLLIN, EPOLLOUT, EPOLLWAKEUP, and > EPOLLET. EPOLLHUP and EPOLLERR can also be specified, > but this is not required: as usual, these events are > always reported if they occur, regardless of whether > they are specified in events. > ? Yes, nothing special here with respect to EPOLLHUP and EPOLLERR. So this looks fine to me. > >>> values in events yield an error. EPOLLEXCLUSIVE may be >>> used only in an EPOLL_CTL_ADD operation; attempts to >>> employ it with EPOLL_CTL_MOD yield an error. If >>> EPOLLEXCLUSIVE has set using epoll_ctl(2), then a subse‐ >>> quent EPOLL_CTL_MOD on the same epfd, fd pair yields an > b>> error. An epoll_ctl(2) that specifies EPOLLEXCLUSIVE in >>> events and specifies the target file descriptor fd as an >>> epoll instance will likewise fail. The error in all of >>> these cases is EINVAL. >>> >>> ERRORS >>> EINVAL An invalid event type was specified along with EPOLLEX‐ >>> CLUSIVE in events. >>> >>> EINVAL op was EPOLL_CTL_MOD and events included EPOLLEXCLUSIVE. >>> >>> EINVAL op was EPOLL_CTL_MOD and the EPOLLEXCLUSIVE flag has >>> previously been applied to this epfd, fd pair. >>> >>> EINVAL EPOLLEXCLUSIVE was specified in event and fd is refers >>> to an epoll instance. > > Returning to the second sentence in this description: > > When a wakeup event occurs and multiple epoll file descrip‐ > tors are attached to the same target file using EPOLLEXCLU‐ > SIVE, one or more of the epoll file descriptors will > receive an event with epoll_wait(2). > > There is a point that is unclear to me: what does "target file" refer to? > Is it an open file description (aka open file table entry) or an inode? > I suspect the former, but it was not clear in your original text. > So from epoll's perspective, the wakeups are associated with a 'wait queue'. So if the open() and subsequent EPOLL_CTL_ADD (which is done via file->poll()) results in adding to the same 'wait queue' then we will get 'exclusive' wakeup behavior. So in general, I think the answer here is that its associated with the inode (I coudn't say with 100% certainty without really looking at all file->poll() implementations). Certainly, with the 'FIFO' example below, the two scenarios will have the same behavior with respect to EPOLLEXCLUSIVE. Also, the 'non-exclusive' mode would be subject to the same question of which wait queue is the epfd is associated with... Thanks, -Jason > To make this point even clearer, here are two scenarios I'm thinking of. > In each case, we're talking of monitoring the read end of a FIFO. > > === > > Scenario 1: > > We have three processes each of which > 1. Creates an epoll instance > 2. Opens the read end of the FIFO > 3. Adds the read end of the FIFO to the epoll instance, specifying > EPOLLEXCLUSIVE > > When input becomes available on the FIFO, how many processes > get a wakeup? > > === > > Scenario 3 > > A parent process opens the read end of a FIFO and then calls > fork() three times to create three children. Each child then: > > 1. Creates an epoll instance > 2. Adds the read end of the FIFO to the epoll instance, specifying > EPOLLEXCLUSIVE > > When input becomes available on the FIFO, how many processes > get a wakeup? > > === > > Cheers, > > Michael > ``` ## Re: \[PATCH\] epoll: add exclusive wakeups flag - From: Michael Kerrisk (man-pages) \ - Date: 2016-03-14 20:01:37 - Message: https://ratatoskr.run/linux-api/2016/03/105827.md ```text Hi Jason, On 03/15/2016 08:32 AM, Jason Baron wrote: > > > On 03/14/2016 01:47 PM, Michael Kerrisk (man-pages) wrote: >> [Restoring CC, which I see I accidentally dropped, one iteration back.] [...] >>>> values in events yield an error. EPOLLEXCLUSIVE may be >>>> used only in an EPOLL_CTL_ADD operation; attempts to >>>> employ it with EPOLL_CTL_MOD yield an error. If >>>> EPOLLEXCLUSIVE has set using epoll_ctl(2), then a subse‐ >>>> quent EPOLL_CTL_MOD on the same epfd, fd pair yields an >> b>> error. An epoll_ctl(2) that specifies EPOLLEXCLUSIVE in >>>> events and specifies the target file descriptor fd as an >>>> epoll instance will likewise fail. The error in all of >>>> these cases is EINVAL. >>>> >>>> ERRORS >>>> EINVAL An invalid event type was specified along with EPOLLEX‐ >>>> CLUSIVE in events. >>>> >>>> EINVAL op was EPOLL_CTL_MOD and events included EPOLLEXCLUSIVE. >>>> >>>> EINVAL op was EPOLL_CTL_MOD and the EPOLLEXCLUSIVE flag has >>>> previously been applied to this epfd, fd pair. >>>> >>>> EINVAL EPOLLEXCLUSIVE was specified in event and fd is refers >>>> to an epoll instance. >> >> Returning to the second sentence in this description: >> >> When a wakeup event occurs and multiple epoll file descrip‐ >> tors are attached to the same target file using EPOLLEXCLU‐ >> SIVE, one or more of the epoll file descriptors will >> receive an event with epoll_wait(2). >> >> There is a point that is unclear to me: what does "target file" refer to? >> Is it an open file description (aka open file table entry) or an inode? >> I suspect the former, but it was not clear in your original text. >> > > So from epoll's perspective, the wakeups are associated with a 'wait > queue'. So if the open() and subsequent EPOLL_CTL_ADD (which is done via > file->poll()) results in adding to the same 'wait queue' then we will > get 'exclusive' wakeup behavior. > > So in general, I think the answer here is that its associated with the > inode (I coudn't say with 100% certainty without really looking at all > file->poll() implementations). Certainly, with the 'FIFO' example below, > the two scenarios will have the same behavior with respect to > EPOLLEXCLUSIVE. So, in both scenarios, *one or more* processes will get a wakeup? (I'll try to add something to the text to clarify the detail we're discussing.) > Also, the 'non-exclusive' mode would be subject to the same question of > which wait queue is the epfd is associated with... I'm not sure of the point you are trying to make here? Cheers, Michael >> To make this point even clearer, here are two scenarios I'm thinking of. >> In each case, we're talking of monitoring the read end of a FIFO. >> >> === >> >> Scenario 1: >> >> We have three processes each of which >> 1. Creates an epoll instance >> 2. Opens the read end of the FIFO >> 3. Adds the read end of the FIFO to the epoll instance, specifying >> EPOLLEXCLUSIVE >> >> When input becomes available on the FIFO, how many processes >> get a wakeup? >> >> === >> >> Scenario 3 >> >> A parent process opens the read end of a FIFO and then calls >> fork() three times to create three children. Each child then: >> >> 1. Creates an epoll instance >> 2. Adds the read end of the FIFO to the epoll instance, specifying >> EPOLLEXCLUSIVE >> >> When input becomes available on the FIFO, how many processes >> get a wakeup? >> >> === >> >> Cheers, >> >> Michael >> > -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html ``` ## Re: \[PATCH\] epoll: add exclusive wakeups flag - From: Michael Kerrisk (man-pages) \ - Date: 2016-03-14 21:03:57 - Message: https://ratatoskr.run/linux-api/2016/03/105826.md ```text Hi Jason, On 03/15/2016 09:01 AM, Michael Kerrisk (man-pages) wrote: > Hi Jason, > > On 03/15/2016 08:32 AM, Jason Baron wrote: >> >> >> On 03/14/2016 01:47 PM, Michael Kerrisk (man-pages) wrote: >>> [Restoring CC, which I see I accidentally dropped, one iteration back.] [...] >>> Returning to the second sentence in this description: >>> >>> When a wakeup event occurs and multiple epoll file descrip‐ >>> tors are attached to the same target file using EPOLLEXCLU‐ >>> SIVE, one or more of the epoll file descriptors will >>> receive an event with epoll_wait(2). >>> >>> There is a point that is unclear to me: what does "target file" refer to? >>> Is it an open file description (aka open file table entry) or an inode? >>> I suspect the former, but it was not clear in your original text. >>> >> >> So from epoll's perspective, the wakeups are associated with a 'wait >> queue'. So if the open() and subsequent EPOLL_CTL_ADD (which is done via >> file->poll()) results in adding to the same 'wait queue' then we will >> get 'exclusive' wakeup behavior. >> >> So in general, I think the answer here is that its associated with the >> inode (I coudn't say with 100% certainty without really looking at all >> file->poll() implementations). Certainly, with the 'FIFO' example below, >> the two scenarios will have the same behavior with respect to >> EPOLLEXCLUSIVE. So, I was actually a little surprised by this, and went away and tested this point. It appears to me that that the two scenarios described below do NOT have the same behavior with respect to EPOLLEXCLUSIVE. See below. > So, in both scenarios, *one or more* processes will get a wakeup? > (I'll try to add something to the text to clarify the detail we're > discussing.) > >> Also, the 'non-exclusive' mode would be subject to the same question of >> which wait queue is the epfd is associated with... > > I'm not sure of the point you are trying to make here? > > Cheers, > > Michael > > >>> To make this point even clearer, here are two scenarios I'm thinking of. >>> In each case, we're talking of monitoring the read end of a FIFO. >>> >>> === >>> >>> Scenario 1: >>> >>> We have three processes each of which >>> 1. Creates an epoll instance >>> 2. Opens the read end of the FIFO >>> 3. Adds the read end of the FIFO to the epoll instance, specifying >>> EPOLLEXCLUSIVE >>> >>> When input becomes available on the FIFO, how many processes >>> get a wakeup? When I test this scenario, all three processes get a wakeup. >>> === >>> >>> Scenario 3 >>> >>> A parent process opens the read end of a FIFO and then calls >>> fork() three times to create three children. Each child then: >>> >>> 1. Creates an epoll instance >>> 2. Adds the read end of the FIFO to the epoll instance, specifying >>> EPOLLEXCLUSIVE >>> >>> When input becomes available on the FIFO, how many processes >>> get a wakeup? When I test this scenario, one process gets a wakeup. In other words, "target file" appears to mean open file description (aka open file table entry), not inode. This is actually what I suspected might be the case, but now I am puzzled. Given what I've discovered and what you suggest are the semantics, is the implementation correct? (I suspect that it is, but it is at odds with your statement above. My test programs are inline below. Cheers, Michael ============ /* t_EPOLLEXCLUSIVE_multipen.c Licensed under GNU GPLv2 or later. */ #include #include #include #include #include #include #include #include #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \ } while (0) #define usageErr(msg, progName) \ do { fprintf(stderr, "Usage: "); \ fprintf(stderr, msg, progName); \ exit(EXIT_FAILURE); } while (0) #ifndef EPOLLEXCLUSIVE #define EPOLLEXCLUSIVE (1 << 28) #endif int main(int argc, char *argv[]) { int fd, epfd, nready; struct epoll_event ev, rev; if (argc != 2 || strcmp(argv[1], "--help") == 0) usageErr("%s n", argv[0]); epfd = epoll_create(2); if (epfd == -1) errExit("epoll_create"); fd = open(argv[1], O_RDONLY); if (fd == -1) errExit("open"); printf("Opened %s\n", argv[1]); ev.events = EPOLLIN | EPOLLEXCLUSIVE; if (epoll_ctl(epfd, EPOLL_CTL_ADD, fd, &ev) == -1) errExit("epoll_ctl"); nready = epoll_wait(epfd, &rev, 1, -1); if (nready == -1) errExit("epoll-wait"); printf("epoll_wait() returned %d\n", nready); exit(EXIT_SUCCESS); } =============== /* t_EPOLLEXCLUSIVE_fork.c Licensed under GNU GPLv2 or later. */ #include #include #include #include #include #include #include #include #include #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \ } while (0) #define usageErr(msg, progName) \ do { fprintf(stderr, "Usage: "); \ fprintf(stderr, msg, progName); \ exit(EXIT_FAILURE); } while (0) #ifndef EPOLLEXCLUSIVE #define EPOLLEXCLUSIVE (1 << 28) #endif int main(int argc, char *argv[]) { int fd, epfd, nready; struct epoll_event ev, rev; int cnum; if (argc != 2 || strcmp(argv[1], "--help") == 0) usageErr("%s n", argv[0]); fd = open(argv[1], O_RDONLY); if (fd == -1) errExit("open"); printf("Opened %s\n", argv[1]); for (cnum = 0; cnum < 3; cnum++) { switch (fork()) { case -1: errExit("fork"); case 0: /* Child */ epfd = epoll_create(2); if (epfd == -1) errExit("epoll_create"); ev.events = EPOLLIN | EPOLLEXCLUSIVE; if (epoll_ctl(epfd, EPOLL_CTL_ADD, fd, &ev) == -1) errExit("epoll_ctl"); nready = epoll_wait(epfd, &rev, 1, -1); if (nready == -1) errExit("epoll-wait"); printf("Child %d: epoll_wait() returned %d\n", cnum, nready); exit(EXIT_SUCCESS); default: break; } } wait(NULL); wait(NULL); wait(NULL); exit(EXIT_SUCCESS); } -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ ``` ## Re: \[PATCH\] epoll: add exclusive wakeups flag - From: Jason Baron \ - Date: 2016-03-14 22:35:10 - Message: https://ratatoskr.run/linux-api/2016/03/105825.md ```text Hi Michael, On 03/14/2016 05:03 PM, Michael Kerrisk (man-pages) wrote: > Hi Jason, > > On 03/15/2016 09:01 AM, Michael Kerrisk (man-pages) wrote: >> Hi Jason, >> >> On 03/15/2016 08:32 AM, Jason Baron wrote: >>> >>> >>> On 03/14/2016 01:47 PM, Michael Kerrisk (man-pages) wrote: >>>> [Restoring CC, which I see I accidentally dropped, one iteration back.] > > [...] > >>>> Returning to the second sentence in this description: >>>> >>>> When a wakeup event occurs and multiple epoll file descrip‐ >>>> tors are attached to the same target file using EPOLLEXCLU‐ >>>> SIVE, one or more of the epoll file descriptors will >>>> receive an event with epoll_wait(2). >>>> >>>> There is a point that is unclear to me: what does "target file" refer to? >>>> Is it an open file description (aka open file table entry) or an inode? >>>> I suspect the former, but it was not clear in your original text. >>>> >>> >>> So from epoll's perspective, the wakeups are associated with a 'wait >>> queue'. So if the open() and subsequent EPOLL_CTL_ADD (which is done via >>> file->poll()) results in adding to the same 'wait queue' then we will >>> get 'exclusive' wakeup behavior. >>> >>> So in general, I think the answer here is that its associated with the >>> inode (I coudn't say with 100% certainty without really looking at all >>> file->poll() implementations). Certainly, with the 'FIFO' example below, >>> the two scenarios will have the same behavior with respect to >>> EPOLLEXCLUSIVE. > > So, I was actually a little surprised by this, and went away and tested > this point. It appears to me that that the two scenarios described below > do NOT have the same behavior with respect to EPOLLEXCLUSIVE. See below. > >> So, in both scenarios, *one or more* processes will get a wakeup? >> (I'll try to add something to the text to clarify the detail we're >> discussing.) >> >>> Also, the 'non-exclusive' mode would be subject to the same question of >>> which wait queue is the epfd is associated with... >> >> I'm not sure of the point you are trying to make here? >> >> Cheers, >> >> Michael >> >> >>>> To make this point even clearer, here are two scenarios I'm thinking of. >>>> In each case, we're talking of monitoring the read end of a FIFO. >>>> >>>> === >>>> >>>> Scenario 1: >>>> >>>> We have three processes each of which >>>> 1. Creates an epoll instance >>>> 2. Opens the read end of the FIFO >>>> 3. Adds the read end of the FIFO to the epoll instance, specifying >>>> EPOLLEXCLUSIVE >>>> >>>> When input becomes available on the FIFO, how many processes >>>> get a wakeup? > > When I test this scenario, all three processes get a wakeup. > >>>> === >>>> >>>> Scenario 3 >>>> >>>> A parent process opens the read end of a FIFO and then calls >>>> fork() three times to create three children. Each child then: >>>> >>>> 1. Creates an epoll instance >>>> 2. Adds the read end of the FIFO to the epoll instance, specifying >>>> EPOLLEXCLUSIVE >>>> >>>> When input becomes available on the FIFO, how many processes >>>> get a wakeup? > > When I test this scenario, one process gets a wakeup. > > In other words, "target file" appears to mean open file description > (aka open file table entry), not inode. > > This is actually what I suspected might be the case, but now I am > puzzled. Given what I've discovered and what you suggest are the > semantics, is the implementation correct? (I suspect that it is, > but it is at odds with your statement above. My test programs are > inline below. > > Cheers, > > Michael > Thanks for the test cases. So in your first test case, you are exiting immediately after the epoll_wait() returns. So this is actually causing the next wakeup. And then the 2nd thread returns from epoll_wait() and this causes the 3rd wakeup. So the wakeups are actually not happening from the write directly, but instead from the readers doing a close(). If you do some sort of sleep after the epoll_wait() you can confirm the behavior. So I believe this is working as expected. Thanks, -Jason > ============ > > /* t_EPOLLEXCLUSIVE_multipen.c > > Licensed under GNU GPLv2 or later. > */ > #include > #include > #include > #include > #include > #include > #include > #include > > #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \ > } while (0) > > #define usageErr(msg, progName) \ > do { fprintf(stderr, "Usage: "); \ > fprintf(stderr, msg, progName); \ > exit(EXIT_FAILURE); } while (0) > > #ifndef EPOLLEXCLUSIVE > #define EPOLLEXCLUSIVE (1 << 28) > #endif > > int > main(int argc, char *argv[]) > { > int fd, epfd, nready; > struct epoll_event ev, rev; > > if (argc != 2 || strcmp(argv[1], "--help") == 0) > usageErr("%s n", argv[0]); > > epfd = epoll_create(2); > if (epfd == -1) > errExit("epoll_create"); > > fd = open(argv[1], O_RDONLY); > if (fd == -1) > errExit("open"); > printf("Opened %s\n", argv[1]); > > ev.events = EPOLLIN | EPOLLEXCLUSIVE; > if (epoll_ctl(epfd, EPOLL_CTL_ADD, fd, &ev) == -1) > errExit("epoll_ctl"); > > nready = epoll_wait(epfd, &rev, 1, -1); > if (nready == -1) > errExit("epoll-wait"); > printf("epoll_wait() returned %d\n", nready); > > exit(EXIT_SUCCESS); > } > > =============== > > /* t_EPOLLEXCLUSIVE_fork.c > > Licensed under GNU GPLv2 or later. > */ > > #include > #include > #include > #include > #include > #include > #include > #include > #include > > #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \ > } while (0) > > #define usageErr(msg, progName) \ > do { fprintf(stderr, "Usage: "); \ > fprintf(stderr, msg, progName); \ > exit(EXIT_FAILURE); } while (0) > > #ifndef EPOLLEXCLUSIVE > #define EPOLLEXCLUSIVE (1 << 28) > #endif > > int > main(int argc, char *argv[]) > { > int fd, epfd, nready; > struct epoll_event ev, rev; > int cnum; > > if (argc != 2 || strcmp(argv[1], "--help") == 0) > usageErr("%s n", argv[0]); > > fd = open(argv[1], O_RDONLY); > if (fd == -1) > errExit("open"); > printf("Opened %s\n", argv[1]); > > for (cnum = 0; cnum < 3; cnum++) { > switch (fork()) { > case -1: > errExit("fork"); > > case 0: /* Child */ > epfd = epoll_create(2); > if (epfd == -1) > errExit("epoll_create"); > > ev.events = EPOLLIN | EPOLLEXCLUSIVE; > if (epoll_ctl(epfd, EPOLL_CTL_ADD, fd, &ev) == -1) > errExit("epoll_ctl"); > > nready = epoll_wait(epfd, &rev, 1, -1); > if (nready == -1) > errExit("epoll-wait"); > printf("Child %d: epoll_wait() returned %d\n", cnum, nready); > exit(EXIT_SUCCESS); > > default: > break; > } > } > > wait(NULL); > wait(NULL); > wait(NULL); > > exit(EXIT_SUCCESS); > } > ``` ## Re: \[PATCH\] epoll: add exclusive wakeups flag - From: Madars Vitolins \ - Date: 2016-03-14 23:09:21 - Message: https://ratatoskr.run/linux-api/2016/03/105824.md ```text Hi Jason and Michael, Hmm... I tried to play with those pipe samples bellow, but even with sleep I got that all process wakeups (maybe I miss something too), also tried with EPOLLIN. On same bases I created sample with Posix Queues with EPOLLIN | EPOLLEXCLUSIVE and the goods news are that it works correctly. file q.c: ================== #include #include #include #include #include #include #include #include #include #include #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \ } while (0) #define usageErr(msg, progName) \ do { fprintf(stderr, "Usage: "); \ fprintf(stderr, msg, progName); \ exit(EXIT_FAILURE); } while (0) #ifndef EPOLLEXCLUSIVE #define EPOLLEXCLUSIVE (1 << 28) #endif #define MAX_SIZE 10 int main (int argc, char *argv[]) { int epfd, nready; struct epoll_event ev, rev; mqd_t fd; struct mq_attr attr; char buffer[MAX_SIZE + 1]; int cnum; /* initialize the queue attributes */ attr.mq_flags = 0; attr.mq_maxmsg = 5; attr.mq_msgsize = MAX_SIZE; attr.mq_curmsgs = 0; /* cleanup for multiple runs... */ mq_unlink ("/TESTQ"); /* create the message queue */ fd = mq_open ("/TESTQ", O_CREAT | O_RDWR | O_NONBLOCK, S_IWUSR | S_IRUSR, &attr); if (fd == -1) errExit ("open"); for (cnum = 0; cnum < 3; cnum++) { switch (fork ()) { case -1: errExit ("fork"); case 0: /* Child */ epfd = epoll_create (2); if (epfd == -1) errExit ("epoll_create"); ev.events = EPOLLIN | EPOLLEXCLUSIVE; if (epoll_ctl (epfd, EPOLL_CTL_ADD, fd, &ev) == -1) errExit ("epoll_ctl"); printf ("About to wait...\n"); nready = epoll_wait (epfd, &rev, 1, -1); if (nready == -1) errExit ("epoll-wait"); printf ("Child %d: epoll_wait() returned %d\n", cnum, nready); exit (EXIT_SUCCESS); default: break; } } sleep (1); /* send a msq to Q */ memset (buffer, 0, MAX_SIZE); if (0 > mq_send (fd, buffer, MAX_SIZE, 0)) errExit ("mq_send"); printf ("msg sent ok...\n"); wait (NULL); wait (NULL); wait (NULL); exit (EXIT_SUCCESS); } ================== $ gcc q.c -lrt $ ./a.out About to wait... About to wait... About to wait... msg sent ok... Child 2: epoll_wait() returned 1 ^C $ Best regards, Madars Jason Baron @ 2016-03-15 00:35 rakstīja: > Hi Michael, > > On 03/14/2016 05:03 PM, Michael Kerrisk (man-pages) wrote: >> Hi Jason, >> >> On 03/15/2016 09:01 AM, Michael Kerrisk (man-pages) wrote: >>> Hi Jason, >>> >>> On 03/15/2016 08:32 AM, Jason Baron wrote: >>>> >>>> >>>> On 03/14/2016 01:47 PM, Michael Kerrisk (man-pages) wrote: >>>>> [Restoring CC, which I see I accidentally dropped, one iteration back.] >> >> [...] >> >>>>> Returning to the second sentence in this description: >>>>> >>>>> When a wakeup event occurs and multiple epoll file descrip‐ >>>>> tors are attached to the same target file using EPOLLEXCLU‐ >>>>> SIVE, one or more of the epoll file descriptors will >>>>> receive an event with epoll_wait(2). >>>>> >>>>> There is a point that is unclear to me: what does "target file" refer to? >>>>> Is it an open file description (aka open file table entry) or an inode? >>>>> I suspect the former, but it was not clear in your original text. >>>>> >>>> >>>> So from epoll's perspective, the wakeups are associated with a 'wait >>>> queue'. So if the open() and subsequent EPOLL_CTL_ADD (which is done via >>>> file->poll()) results in adding to the same 'wait queue' then we will >>>> get 'exclusive' wakeup behavior. >>>> >>>> So in general, I think the answer here is that its associated with the >>>> inode (I coudn't say with 100% certainty without really looking at all >>>> file->poll() implementations). Certainly, with the 'FIFO' example below, >>>> the two scenarios will have the same behavior with respect to >>>> EPOLLEXCLUSIVE. >> >> So, I was actually a little surprised by this, and went away and tested >> this point. It appears to me that that the two scenarios described below >> do NOT have the same behavior with respect to EPOLLEXCLUSIVE. See below. >> >>> So, in both scenarios, *one or more* processes will get a wakeup? >>> (I'll try to add something to the text to clarify the detail we're >>> discussing.) >>> >>>> Also, the 'non-exclusive' mode would be subject to the same question of >>>> which wait queue is the epfd is associated with... >>> >>> I'm not sure of the point you are trying to make here? >>> >>> Cheers, >>> >>> Michael >>> >>> >>>>> To make this point even clearer, here are two scenarios I'm thinking of. >>>>> In each case, we're talking of monitoring the read end of a FIFO. >>>>> >>>>> === >>>>> >>>>> Scenario 1: >>>>> >>>>> We have three processes each of which >>>>> 1. Creates an epoll instance >>>>> 2. Opens the read end of the FIFO >>>>> 3. Adds the read end of the FIFO to the epoll instance, specifying >>>>> EPOLLEXCLUSIVE >>>>> >>>>> When input becomes available on the FIFO, how many processes >>>>> get a wakeup? >> >> When I test this scenario, all three processes get a wakeup. >> >>>>> === >>>>> >>>>> Scenario 3 >>>>> >>>>> A parent process opens the read end of a FIFO and then calls >>>>> fork() three times to create three children. Each child then: >>>>> >>>>> 1. Creates an epoll instance >>>>> 2. Adds the read end of the FIFO to the epoll instance, specifying >>>>> EPOLLEXCLUSIVE >>>>> >>>>> When input becomes available on the FIFO, how many processes >>>>> get a wakeup? >> >> When I test this scenario, one process gets a wakeup. >> >> In other words, "target file" appears to mean open file description >> (aka open file table entry), not inode. >> >> This is actually what I suspected might be the case, but now I am >> puzzled. Given what I've discovered and what you suggest are the >> semantics, is the implementation correct? (I suspect that it is, >> but it is at odds with your statement above. My test programs are >> inline below. >> >> Cheers, >> >> Michael >> > > Thanks for the test cases. So in your first test case, you are exiting > immediately after the epoll_wait() returns. So this is actually causing > the next wakeup. And then the 2nd thread returns from epoll_wait() and > this causes the 3rd wakeup. > > So the wakeups are actually not happening from the write directly, but > instead from the readers doing a close(). If you do some sort of sleep > after the epoll_wait() you can confirm the behavior. So I believe this > is working as expected. > > Thanks, > > -Jason > > >> ============ >> >> /* t_EPOLLEXCLUSIVE_multipen.c >> >> Licensed under GNU GPLv2 or later. >> */ >> #include >> #include >> #include >> #include >> #include >> #include >> #include >> #include >> >> #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \ >> } while (0) >> >> #define usageErr(msg, progName) \ >> do { fprintf(stderr, "Usage: "); \ >> fprintf(stderr, msg, progName); \ >> exit(EXIT_FAILURE); } while (0) >> >> #ifndef EPOLLEXCLUSIVE >> #define EPOLLEXCLUSIVE (1 << 28) >> #endif >> >> int >> main(int argc, char *argv[]) >> { >> int fd, epfd, nready; >> struct epoll_event ev, rev; >> >> if (argc != 2 || strcmp(argv[1], "--help") == 0) >> usageErr("%s n", argv[0]); >> >> epfd = epoll_create(2); >> if (epfd == -1) >> errExit("epoll_create"); >> >> fd = open(argv[1], O_RDONLY); >> if (fd == -1) >> errExit("open"); >> printf("Opened %s\n", argv[1]); >> >> ev.events = EPOLLIN | EPOLLEXCLUSIVE; >> if (epoll_ctl(epfd, EPOLL_CTL_ADD, fd, &ev) == -1) >> errExit("epoll_ctl"); >> >> nready = epoll_wait(epfd, &rev, 1, -1); >> if (nready == -1) >> errExit("epoll-wait"); >> printf("epoll_wait() returned %d\n", nready); >> >> exit(EXIT_SUCCESS); >> } >> >> =============== >> >> /* t_EPOLLEXCLUSIVE_fork.c >> >> Licensed under GNU GPLv2 or later. >> */ >> >> #include >> #include >> #include >> #include >> #include >> #include >> #include >> #include >> #include >> >> #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \ >> } while (0) >> >> #define usageErr(msg, progName) \ >> do { fprintf(stderr, "Usage: "); \ >> fprintf(stderr, msg, progName); \ >> exit(EXIT_FAILURE); } while (0) >> >> #ifndef EPOLLEXCLUSIVE >> #define EPOLLEXCLUSIVE (1 << 28) >> #endif >> >> int >> main(int argc, char *argv[]) >> { >> int fd, epfd, nready; >> struct epoll_event ev, rev; >> int cnum; >> >> if (argc != 2 || strcmp(argv[1], "--help") == 0) >> usageErr("%s n", argv[0]); >> >> fd = open(argv[1], O_RDONLY); >> if (fd == -1) >> errExit("open"); >> printf("Opened %s\n", argv[1]); >> >> for (cnum = 0; cnum < 3; cnum++) { >> switch (fork()) { >> case -1: >> errExit("fork"); >> >> case 0: /* Child */ >> epfd = epoll_create(2); >> if (epfd == -1) >> errExit("epoll_create"); >> >> ev.events = EPOLLIN | EPOLLEXCLUSIVE; >> if (epoll_ctl(epfd, EPOLL_CTL_ADD, fd, &ev) == -1) >> errExit("epoll_ctl"); >> >> nready = epoll_wait(epfd, &rev, 1, -1); >> if (nready == -1) >> errExit("epoll-wait"); >> printf("Child %d: epoll_wait() returned %d\n", cnum, nready); >> exit(EXIT_SUCCESS); >> >> default: >> break; >> } >> } >> >> wait(NULL); >> wait(NULL); >> wait(NULL); >> >> exit(EXIT_SUCCESS); >> } >> ``` ## Re: \[PATCH\] epoll: add exclusive wakeups flag - From: Michael Kerrisk (man-pages) \ - Date: 2016-03-14 23:27:01 - Message: https://ratatoskr.run/linux-api/2016/03/105823.md ```text Hi Jason, On 03/15/2016 11:35 AM, Jason Baron wrote: > Hi Michael, > > On 03/14/2016 05:03 PM, Michael Kerrisk (man-pages) wrote: >> Hi Jason, >> >> On 03/15/2016 09:01 AM, Michael Kerrisk (man-pages) wrote: >>> Hi Jason, >>> >>> On 03/15/2016 08:32 AM, Jason Baron wrote: >>>> >>>> >>>> On 03/14/2016 01:47 PM, Michael Kerrisk (man-pages) wrote: >>>>> [Restoring CC, which I see I accidentally dropped, one iteration back.] >> >> [...] >> >>>>> Returning to the second sentence in this description: >>>>> >>>>> When a wakeup event occurs and multiple epoll file descrip‐ >>>>> tors are attached to the same target file using EPOLLEXCLU‐ >>>>> SIVE, one or more of the epoll file descriptors will >>>>> receive an event with epoll_wait(2). >>>>> >>>>> There is a point that is unclear to me: what does "target file" refer to? >>>>> Is it an open file description (aka open file table entry) or an inode? >>>>> I suspect the former, but it was not clear in your original text. >>>>> >>>> >>>> So from epoll's perspective, the wakeups are associated with a 'wait >>>> queue'. So if the open() and subsequent EPOLL_CTL_ADD (which is done via >>>> file->poll()) results in adding to the same 'wait queue' then we will >>>> get 'exclusive' wakeup behavior. >>>> >>>> So in general, I think the answer here is that its associated with the >>>> inode (I coudn't say with 100% certainty without really looking at all >>>> file->poll() implementations). Certainly, with the 'FIFO' example below, >>>> the two scenarios will have the same behavior with respect to >>>> EPOLLEXCLUSIVE. >> >> So, I was actually a little surprised by this, and went away and tested >> this point. It appears to me that that the two scenarios described below >> do NOT have the same behavior with respect to EPOLLEXCLUSIVE. See below. >> >>> So, in both scenarios, *one or more* processes will get a wakeup? >>> (I'll try to add something to the text to clarify the detail we're >>> discussing.) >>> >>>> Also, the 'non-exclusive' mode would be subject to the same question of >>>> which wait queue is the epfd is associated with... >>> >>> I'm not sure of the point you are trying to make here? >>> >>> Cheers, >>> >>> Michael >>> >>> >>>>> To make this point even clearer, here are two scenarios I'm thinking of. >>>>> In each case, we're talking of monitoring the read end of a FIFO. >>>>> >>>>> === >>>>> >>>>> Scenario 1: >>>>> >>>>> We have three processes each of which >>>>> 1. Creates an epoll instance >>>>> 2. Opens the read end of the FIFO >>>>> 3. Adds the read end of the FIFO to the epoll instance, specifying >>>>> EPOLLEXCLUSIVE >>>>> >>>>> When input becomes available on the FIFO, how many processes >>>>> get a wakeup? >> >> When I test this scenario, all three processes get a wakeup. >> >>>>> === >>>>> >>>>> Scenario 3 >>>>> >>>>> A parent process opens the read end of a FIFO and then calls >>>>> fork() three times to create three children. Each child then: >>>>> >>>>> 1. Creates an epoll instance >>>>> 2. Adds the read end of the FIFO to the epoll instance, specifying >>>>> EPOLLEXCLUSIVE >>>>> >>>>> When input becomes available on the FIFO, how many processes >>>>> get a wakeup? >> >> When I test this scenario, one process gets a wakeup. >> >> In other words, "target file" appears to mean open file description >> (aka open file table entry), not inode. >> >> This is actually what I suspected might be the case, but now I am >> puzzled. Given what I've discovered and what you suggest are the >> semantics, is the implementation correct? (I suspect that it is, >> but it is at odds with your statement above. My test programs are >> inline below. >> >> Cheers, >> >> Michael >> > > Thanks for the test cases. So in your first test case, you are exiting > immediately after the epoll_wait() returns. So this is actually causing > the next wakeup. Can I just check my understanding of the rationale for the preceding point. The next process is getting woken up, because the previous process did not "consume" the event (that is, the input is still available on the FIFO). Right? > And then the 2nd thread returns from epoll_wait() and > this causes the 3rd wakeup. I added the sleep() calls, but still things don't seem to happen quite as you suggest. In the first scenario, after the first process terminates, *all* of the remaining processes wake from epoll_wait(). What's happening in this case? (This smells like a possible bug.) In the second scenario (fork()), after the first process terminates (without consuming the FIFO input), all of the other processes remain blocked in epoll-wait(). (Note, I extended the test program here to allow the number of child processes to be specified as a command-line argument.) I think I can make sense of that: it's because the open file descriptor for the read end of the FIFO has been duplicated in all of the child processes, and closing the FD in one child does not cause the corresponding open file description in other processes to be torn down because there are other FDs that still refer to it. > So the wakeups are actually not happening from the write directly, but > instead from the readers doing a close(). If you do some sort of sleep > after the epoll_wait() you can confirm the behavior. So I believe this > is working as expected. As note above, I'm still slightly puzzled. Revised test programs pasted below. Cheers, Michael ========== /* t_EPOLLEXCLUSIVE_multiopen.c Licensed under GNU GPLv2 or later. */ #include #include #include #include #include #include #include #include #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \ } while (0) #define usageErr(msg, progName) \ do { fprintf(stderr, "Usage: "); \ fprintf(stderr, msg, progName); \ exit(EXIT_FAILURE); } while (0) #ifndef EPOLLEXCLUSIVE #define EPOLLEXCLUSIVE (1 << 28) #endif int main(int argc, char *argv[]) { int fd, epfd, nready; struct epoll_event ev, rev; if (argc != 2 || strcmp(argv[1], "--help") == 0) usageErr("%s \n", argv[0]); epfd = epoll_create(2); if (epfd == -1) errExit("epoll_create"); fd = open(argv[1], O_RDONLY); if (fd == -1) errExit("open"); printf("Opened %s\n", argv[1]); ev.events = EPOLLIN | EPOLLEXCLUSIVE; if (epoll_ctl(epfd, EPOLL_CTL_ADD, fd, &ev) == -1) errExit("epoll_ctl"); nready = epoll_wait(epfd, &rev, 1, -1); if (nready == -1) errExit("epoll-wait"); printf("epoll_wait() returned %d\n", nready); printf("sleeping\n"); sleep(3); printf("Terminating\n"); exit(EXIT_SUCCESS); } =================== /* t_EPOLLEXCLUSIVE_fork.c Licensed under GNU GPLv2 or later. */ #include #include #include #include #include #include #include #include #include #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \ } while (0) #define usageErr(msg, progName) \ do { fprintf(stderr, "Usage: "); \ fprintf(stderr, msg, progName); \ exit(EXIT_FAILURE); } while (0) #ifndef EPOLLEXCLUSIVE #define EPOLLEXCLUSIVE (1 << 28) #endif int main(int argc, char *argv[]) { int fd, epfd, nready; struct epoll_event ev, rev; int cnum, cmax; if (argc < 2 || strcmp(argv[1], "--help") == 0) usageErr("%s [num-children]\n", argv[0]); fd = open(argv[1], O_RDONLY); if (fd == -1) errExit("open"); printf("Opened %s\n", argv[1]); cmax = (argc > 2) ? atoi(argv[2]) : 3; for (cnum = 0; cnum < cmax; cnum++) { switch (fork()) { case -1: errExit("fork"); case 0: /* Child */ epfd = epoll_create(2); if (epfd == -1) errExit("epoll_create"); ev.events = EPOLLIN | EPOLLEXCLUSIVE; if (epoll_ctl(epfd, EPOLL_CTL_ADD, fd, &ev) == -1) errExit("epoll_ctl"); nready = epoll_wait(epfd, &rev, 1, -1); if (nready == -1) errExit("epoll-wait"); printf("Child %d: epoll_wait() returned %d\n", cnum, nready); printf("sleeping\n"); sleep(3); printf("Child %d terminating\n", cnum); exit(EXIT_SUCCESS); default: break; } } for (cnum = 0; cnum < cmax; cnum++) wait(NULL); exit(EXIT_SUCCESS); } ``` ## Re: \[PATCH\] epoll: add exclusive wakeups flag - From: Jason Baron \ - Date: 2016-03-15 02:36:09 - Message: https://ratatoskr.run/linux-api/2016/03/105822.md ```text Hi Michael, On 03/14/2016 07:26 PM, Michael Kerrisk (man-pages) wrote: > Hi Jason, > > On 03/15/2016 11:35 AM, Jason Baron wrote: >> Hi Michael, >> >> On 03/14/2016 05:03 PM, Michael Kerrisk (man-pages) wrote: >>> Hi Jason, >>> >>> On 03/15/2016 09:01 AM, Michael Kerrisk (man-pages) wrote: >>>> Hi Jason, >>>> >>>> On 03/15/2016 08:32 AM, Jason Baron wrote: >>>>> >>>>> >>>>> On 03/14/2016 01:47 PM, Michael Kerrisk (man-pages) wrote: >>>>>> [Restoring CC, which I see I accidentally dropped, one iteration back.] >>> >>> [...] >>> >>>>>> Returning to the second sentence in this description: >>>>>> >>>>>> When a wakeup event occurs and multiple epoll file descrip‐ >>>>>> tors are attached to the same target file using EPOLLEXCLU‐ >>>>>> SIVE, one or more of the epoll file descriptors will >>>>>> receive an event with epoll_wait(2). >>>>>> >>>>>> There is a point that is unclear to me: what does "target file" refer to? >>>>>> Is it an open file description (aka open file table entry) or an inode? >>>>>> I suspect the former, but it was not clear in your original text. >>>>>> >>>>> >>>>> So from epoll's perspective, the wakeups are associated with a 'wait >>>>> queue'. So if the open() and subsequent EPOLL_CTL_ADD (which is done via >>>>> file->poll()) results in adding to the same 'wait queue' then we will >>>>> get 'exclusive' wakeup behavior. >>>>> >>>>> So in general, I think the answer here is that its associated with the >>>>> inode (I coudn't say with 100% certainty without really looking at all >>>>> file->poll() implementations). Certainly, with the 'FIFO' example below, >>>>> the two scenarios will have the same behavior with respect to >>>>> EPOLLEXCLUSIVE. >>> >>> So, I was actually a little surprised by this, and went away and tested >>> this point. It appears to me that that the two scenarios described below >>> do NOT have the same behavior with respect to EPOLLEXCLUSIVE. See below. >>> >>>> So, in both scenarios, *one or more* processes will get a wakeup? >>>> (I'll try to add something to the text to clarify the detail we're >>>> discussing.) >>>> >>>>> Also, the 'non-exclusive' mode would be subject to the same question of >>>>> which wait queue is the epfd is associated with... >>>> >>>> I'm not sure of the point you are trying to make here? >>>> >>>> Cheers, >>>> >>>> Michael >>>> >>>> >>>>>> To make this point even clearer, here are two scenarios I'm thinking of. >>>>>> In each case, we're talking of monitoring the read end of a FIFO. >>>>>> >>>>>> === >>>>>> >>>>>> Scenario 1: >>>>>> >>>>>> We have three processes each of which >>>>>> 1. Creates an epoll instance >>>>>> 2. Opens the read end of the FIFO >>>>>> 3. Adds the read end of the FIFO to the epoll instance, specifying >>>>>> EPOLLEXCLUSIVE >>>>>> >>>>>> When input becomes available on the FIFO, how many processes >>>>>> get a wakeup? >>> >>> When I test this scenario, all three processes get a wakeup. >>> >>>>>> === >>>>>> >>>>>> Scenario 3 >>>>>> >>>>>> A parent process opens the read end of a FIFO and then calls >>>>>> fork() three times to create three children. Each child then: >>>>>> >>>>>> 1. Creates an epoll instance >>>>>> 2. Adds the read end of the FIFO to the epoll instance, specifying >>>>>> EPOLLEXCLUSIVE >>>>>> >>>>>> When input becomes available on the FIFO, how many processes >>>>>> get a wakeup? >>> >>> When I test this scenario, one process gets a wakeup. >>> >>> In other words, "target file" appears to mean open file description >>> (aka open file table entry), not inode. >>> >>> This is actually what I suspected might be the case, but now I am >>> puzzled. Given what I've discovered and what you suggest are the >>> semantics, is the implementation correct? (I suspect that it is, >>> but it is at odds with your statement above. My test programs are >>> inline below. >>> >>> Cheers, >>> >>> Michael >>> >> >> Thanks for the test cases. So in your first test case, you are exiting >> immediately after the epoll_wait() returns. So this is actually causing >> the next wakeup. > > Can I just check my understanding of the rationale for the preceding > point. The next process is getting woken up, because the previous process > did not "consume" the event (that is, the input is still available on the > FIFO). Right? > >> And then the 2nd thread returns from epoll_wait() and >> this causes the 3rd wakeup. > > I added the sleep() calls, but still things don't seem to happen > quite as you suggest. In the first scenario, after the first process > terminates, *all* of the remaining processes wake from epoll_wait(). > What's happening in this case? (This smells like a possible bug.) Yes, you are right. When the first process exits() and thus closes the read-side of the pipe, it will wake up all the other calls that are in epoll_wait(). When the file closes, since there are no more fds referencing it, the pipe_release() routine does a wakeup specifying both POLLIN and POLLOUT. In this case, all of the epoll exclusive waiters will get a wakeup. The combination of POLLIN and POLLOUT is not expected to be the typical use-case here. Normally, we would have the threads in event loops, and just POLLIN would be set resulting in the exclusive waskeup behavior. So yes, there can be multiple wakeups in some cases, but the *common* case of only POLLIN or only POLLOUT set will yield exclusive wakeups. There is no guarantee here that only 1 thread wakes up - only that *at least* one. > > In the second scenario (fork()), after the first process terminates > (without consuming the FIFO input), all of the other processes remain > blocked in epoll-wait(). (Note, I extended the test program here > to allow the number of child processes to be specified as a command-line > argument.) I think I can make sense of that: it's because the open > file descriptor for the read end of the FIFO has been duplicated > in all of the child processes, and closing the FD in one child > does not cause the corresponding open file description in other > processes to be torn down because there are other FDs that still > refer to it. > Yes, exactly. The final process is going to invoke the pipe_release(), but at that point there is nobody left to wakeup. >> So the wakeups are actually not happening from the write directly, but >> instead from the readers doing a close(). If you do some sort of sleep >> after the epoll_wait() you can confirm the behavior. So I believe this >> is working as expected. > > As note above, I'm still slightly puzzled. > Revised test programs pasted below. > Ok, hopefully this makes sense. Thanks, -Jason > Cheers, > > Michael > > ========== > > /* t_EPOLLEXCLUSIVE_multiopen.c > > Licensed under GNU GPLv2 or later. > */ > > #include > #include > #include > #include > #include > #include > #include > #include > > #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \ > } while (0) > > #define usageErr(msg, progName) \ > do { fprintf(stderr, "Usage: "); \ > fprintf(stderr, msg, progName); \ > exit(EXIT_FAILURE); } while (0) > > #ifndef EPOLLEXCLUSIVE > #define EPOLLEXCLUSIVE (1 << 28) > #endif > > int > main(int argc, char *argv[]) > { > int fd, epfd, nready; > struct epoll_event ev, rev; > > if (argc != 2 || strcmp(argv[1], "--help") == 0) > usageErr("%s \n", argv[0]); > > epfd = epoll_create(2); > if (epfd == -1) > errExit("epoll_create"); > > fd = open(argv[1], O_RDONLY); > if (fd == -1) > errExit("open"); > printf("Opened %s\n", argv[1]); > > ev.events = EPOLLIN | EPOLLEXCLUSIVE; > if (epoll_ctl(epfd, EPOLL_CTL_ADD, fd, &ev) == -1) > errExit("epoll_ctl"); > > nready = epoll_wait(epfd, &rev, 1, -1); > if (nready == -1) > errExit("epoll-wait"); > printf("epoll_wait() returned %d\n", nready); > > printf("sleeping\n"); > sleep(3); > printf("Terminating\n"); > exit(EXIT_SUCCESS); > } > > =================== > > /* t_EPOLLEXCLUSIVE_fork.c > > Licensed under GNU GPLv2 or later. > */ > > #include > #include > #include > #include > #include > #include > #include > #include > #include > > #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \ > } while (0) > > #define usageErr(msg, progName) \ > do { fprintf(stderr, "Usage: "); \ > fprintf(stderr, msg, progName); \ > exit(EXIT_FAILURE); } while (0) > > #ifndef EPOLLEXCLUSIVE > #define EPOLLEXCLUSIVE (1 << 28) > #endif > > int > main(int argc, char *argv[]) > { > int fd, epfd, nready; > struct epoll_event ev, rev; > int cnum, cmax; > > if (argc < 2 || strcmp(argv[1], "--help") == 0) > usageErr("%s [num-children]\n", argv[0]); > > fd = open(argv[1], O_RDONLY); > if (fd == -1) > errExit("open"); > printf("Opened %s\n", argv[1]); > > cmax = (argc > 2) ? atoi(argv[2]) : 3; > > for (cnum = 0; cnum < cmax; cnum++) { > switch (fork()) { > case -1: > errExit("fork"); > > case 0: /* Child */ > epfd = epoll_create(2); > if (epfd == -1) > errExit("epoll_create"); > > ev.events = EPOLLIN | EPOLLEXCLUSIVE; > if (epoll_ctl(epfd, EPOLL_CTL_ADD, fd, &ev) == -1) > errExit("epoll_ctl"); > > nready = epoll_wait(epfd, &rev, 1, -1); > if (nready == -1) > errExit("epoll-wait"); > printf("Child %d: epoll_wait() returned %d\n", cnum, nready); > printf("sleeping\n"); > sleep(3); > printf("Child %d terminating\n", cnum); > exit(EXIT_SUCCESS); > > default: > break; > } > } > > for (cnum = 0; cnum < cmax; cnum++) > wait(NULL); > > exit(EXIT_SUCCESS); > } > ```