Messages in this thread | | | Subject | Re: [BUG] Regression on behavior of EPOLLET | EPOLLIN for AF_UNIX sockets in 3.2 | From | Eric Dumazet <> | Date | Fri, 27 Jan 2012 18:53:42 +0100 |
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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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