lkml.org 
[lkml]   [2012]   [Jan]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    SubjectRe: [BUG] Regression on behavior of EPOLLET | EPOLLIN for AF_UNIX sockets in 3.2
    From
    Date
    Le vendredi 27 janvier 2012 à 12:05 -0500, Nick Mathewson a écrit :
    > [1.] One line summary of the problem:
    >
    > EPOLLET doesn't give edge-triggered behavior for AF_UNIX sockets in 3.2
    >
    > [2.] Full description of the problem/report:
    >
    > When epoll is told to listen to a readable socket with the flags
    > EPOLLIN|EPOLLET, it is supposed to report the event once, and then
    > not report the event again until the socket has first become
    > non-readable and then become readable again. (This behavior is part
    > of the definition of edge-triggered events, IIUC.)
    >
    > But with AF_UNIX sockets on Linux 3.2, a call to read() on a socket
    > that does not drain the socket's buffer completely can apparently
    > cause epoll to think that the socket has generated another event,
    > even if no further data has actually arrived at the socket.
    >
    > This behavior did not occur in 3.1, and does not occur in 3.2 with
    > AF_INET sockets or with pipes.
    >
    > [3.] Keywords:
    >
    > networking, AF_UNIX, epoll, socket
    >
    > [4.] Kernel version (from /proc/version):
    >
    > First found in:
    >
    > Linux version 3.2.1-3.fc16.x86_64
    > (mockbuild@x86-13.phx2.fedoraproject.org) (gcc version 4.6.2 20111027
    > (Red Hat 4.6.2-1) (GCC) ) #1 SMP Mon Jan 23 15:36:17 UTC 2012
    >
    > Another user has reproduced this with:
    >
    > Linux version 3.2.0-1-686-pae (Debian 3.2.1-1) (ben@decadent.org.uk)
    > (gcc version 4.6.2 (Debian 4.6.2-11) ) #1 SMP Thu Jan 19 10:56:51 UTC
    > 2012
    >
    > [6.] A small shell script or example program which triggers the
    > problem (if possible)
    >
    > #include <sys/epoll.h>
    > #include <sys/types.h>
    > #include <sys/socket.h>
    > #include <unistd.h>
    > #include <fcntl.h>
    >
    > #include <stdio.h>
    > #include <errno.h>
    > #include <string.h>
    >
    > int
    > main(int argc, const char **argv)
    > {
    > int epfd;
    > int pair[2];
    > struct epoll_event epev;
    > int n, r, n_reads;
    >
    > if ((epfd = epoll_create(32)) < 0) {
    > perror("epoll_create()");
    > return 2;
    > }
    > if (socketpair(AF_UNIX, SOCK_STREAM, 0, pair) < 0) {
    > perror("socketpair()");
    > return 2;
    > }
    >
    > if (fcntl(pair[0], F_SETFL, O_NONBLOCK) < 0) {
    > perror("fcntl()");
    > return 2;
    > }
    >
    > memset(&epev, 0, sizeof(epev));
    > epev.events = EPOLLIN | EPOLLET;
    > epev.data.fd = pair[0];
    > if (epoll_ctl(epfd, EPOLL_CTL_ADD, pair[0], &epev) < 0) {
    > perror("epoll_ctl()");
    > return 2;
    > }
    >
    > if ((n = write(pair[1], "A 21-character string", 21)) < 0) {
    > perror("write()");
    > return 2;
    > }
    >
    > /* pair[0] should now be readable. EPOLLET above has said that we
    > * want edge-triggered behavior, so we should only get a single
    > * EPOLLIN event on the socket. But on Linux 3.2, for some reason,
    > * reading a single byte from the socket causes us to get another
    > * EPOLLIN event.
    > */
    > n_reads = 0;
    > while ((r = epoll_wait(epfd, &epev, 1, 500)) == 1) {
    > char byte[1];
    > printf("epoll_wait() said: events=%d, fd=%d\n",
    > epev.events, epev.data.fd);
    > n = read(pair[0], byte, 1);
    > if (n < 0 && errno == EAGAIN) {
    > puts("read() reported EAGAIN.");
    > } else if (n < 0) {
    > perror("read()");
    > } else if (n == 0) {
    > puts("read() reported EOF.");
    > } else {
    > printf("Read %d byte(s)\n", n);
    > ++n_reads;
    > }
    > }
    > if (r == 0) {
    > puts("Timeout without event.");
    > } else {
    > perror("epoll_wait()");
    > }
    >
    > close(pair[0]);
    > close(pair[1]);
    > close(epfd);
    >
    > if (n_reads == 1) {
    > puts("Exactly one read event. Good.");
    > } else {
    > printf("Got %d read events. That's not right!\n", n_reads);
    > }
    > return (n_reads == 1) ? 0 : 1;
    > }
    > --

    Hi

    Probably coming from commit 0884d7aa24e15e72b3c07f7da910a13bb7df3592
    (AF_UNIX: Fix poll blocking problem when reading from a stream socket)

    When we requeue skb because not completely eaten, we call again

    sk->sk_data_ready(sk, skb->len);



    --
    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to majordomo@vger.kernel.org
    More majordomo info at http://vger.kernel.org/majordomo-info.html
    Please read the FAQ at http://www.tux.org/lkml/

    \
     
     \ /
      Last update: 2012-01-27 18:55    [W:0.045 / U:31.564 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site