Regression: epoll edge-triggered (EPOLLET) for pipes/FIFOs

[Michael Kerrisk (man-pages)]

Hello Linus,

Between Linux 5.4 and 5.5 a regression was introduced in the operation
of the epoll EPOLLET flag. From some manual bisecting, the regression
appears to have been introduced in

         commit 1b6b26ae7053e4914181eedf70f2d92c12abda8a
         Author: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
         Date:   Sat Dec 7 12:14:28 2019 -0800

             pipe: fix and clarify pipe write wakeup logic

(I also built a kernel from the  immediate preceding commit, and did
not observe the regression.)

The aim of ET (edge-triggered) notification is that epoll_wait() will
tell us a file descriptor is ready only if there has been new activity
on the FD since we were last informed about the FD. So, in the
following scenario where the read end of a pipe is being monitored
with EPOLLET, we see:

[Write a byte to write end of pipe]
1. Call epoll_wait() ==> tells us pipe read end is ready
2. Call epoll_wait() [again] ==> does not tell us that the read end of
pipe is ready

    (By contrast, in step 2, level-triggered notification would tell
    us the read end of the pipe is read.)

If we go further:

[Write another byte to write end of pipe]
3. Call epoll_wait() ==> tells us pipe read end is ready

The above was true until the regression. Now, step 3 does not tell us
that the pipe read end is ready, even though there is NEW input
available on the pipe. (In the analogous situation for sockets and
terminals, step 3 does (still) correctly tell us that the FD is
ready.)

I've appended a test program below. The following are the results on
kernel 5.4.0:

        $ ./pipe_epollet_test
        Writing a byte to pipe()
            1: OK:   ret = 1, events = [ EPOLLIN ]
            2: OK:   ret = 0
        Writing a byte to pipe()
            3: OK:   ret = 1, events = [ EPOLLIN ]
        Closing write end of pipe()
            4: OK:   ret = 1, events = [ EPOLLIN EPOLLHUP ]

On current kernels, the results are as follows:

        $ ./pipe_epollet_test
        Writing a byte to pipe()
            1: OK:   ret = 1, events = [ EPOLLIN ]
            2: OK:   ret = 0
        Writing a byte to pipe()
            3: FAIL: ret = 0; EXPECTED: ret = 1, events = [ EPOLLIN ]
        Closing write end of pipe()
            4: OK:   ret = 1, events = [ EPOLLIN EPOLLHUP ]

Thanks,

Michael

=====

/* pipe_epollet_test.c

   Copyright (c) 2020, Michael Kerrisk <mtk.manpages@xxxxxxxxx>

   Licensed under GNU GPLv2 or later.
*/
#include <sys/epoll.h>
#include <stdio.h>
#include <stdbool.h>
#include <stdlib.h>
#include <unistd.h>

#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                        } while (0)

static void
printMask(int events)
{
    printf(" [ %s%s]",
                (events & EPOLLIN)  ? "EPOLLIN "  : "",
                (events & EPOLLHUP) ? "EPOLLHUP " : "");
}

static void
doEpollWait(int epfd, int timeout, int expectedRetval, int expectedEvents)
{
    struct epoll_event ev;
    static int callNum = 0;

    int retval = epoll_wait(epfd, &ev, 1, timeout);
    if (retval == -1) {
        perror("epoll_wait");
        return;
    }

    /* The test succeeded if (1) we got the expected return value and
       (2) when the return value was 1, we got the expected events mask */

    bool succeeded = retval == expectedRetval &&
            (expectedRetval == 0 || expectedEvents == ev.events);

    callNum++;
    printf("    %d: ", callNum);

    if (succeeded)
        printf("OK:   ");
    else
        printf("FAIL: ");

    printf("ret = %d", retval);

    if (retval == 1) {
        printf(", events =");
        printMask(ev.events);
    }

    if (!succeeded) {
        printf("; EXPECTED: ret = %d", expectedRetval);
        if (expectedRetval == 1) {
            printf(", events =");
            printMask(expectedEvents);
        }
    }
    printf("\n");
}

int
main(int argc, char *argv[])
{
    int epfd;
    int pfd[2];

    epfd = epoll_create(1);
    if (epfd == -1)
        errExit("epoll_create");

    /* Create a pipe and add read end to epoll interest list */

    if (pipe(pfd) == -1)
        errExit("pipe");

    struct epoll_event ev;
    ev.data.fd = pfd[0];
    ev.events = EPOLLIN | EPOLLET;
    if (epoll_ctl(epfd, EPOLL_CTL_ADD, pfd[0], &ev) == -1)
        errExit("epoll_ctl");

    /* Run some tests */

    printf("Writing a byte to pipe()\n");
    write(pfd[1], "a", 1);

    doEpollWait(epfd, 0, 1, EPOLLIN);
    doEpollWait(epfd, 0, 0, 0);

    printf("Writing a byte to pipe()\n");
    write(pfd[1], "a", 1);

    doEpollWait(epfd, 0, 1, EPOLLIN);

    printf("Closing write end of pipe()\n");
    close(pfd[1]);

    doEpollWait(epfd, 0, 1, EPOLLIN | EPOLLHUP);

    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/