Re: [PATCH] epoll: add exclusive wakeups flag

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:

quoted

On 03/14/2016 01:47 PM, Michael Kerrisk (man-pages) wrote:

quoted

[Restoring CC, which I see I accidentally dropped, one iteration back.]

[...]

quoted

quoted

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.)

quoted

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

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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.

quoted

quoted

===

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 <sys/epoll.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/types.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>

#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 <FIFO>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 <sys/epoll.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>

#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 <FIFO>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/