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SubjectRe: [BUG] Regression on behavior of EPOLLET | EPOLLIN for AF_UNIX sockets in 3.2
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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);



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