[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:
quoted
On 03/11/2016 09:51 PM, Jason Baron wrote:
quoted
On 03/11/2016 03:30 PM, Michael Kerrisk (man-pages) wrote:
[...]
Hi Michael,
Looks good. One comment below.
Thanks,
quoted
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.
?
quoted
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
quoted
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/