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SubjectRe: [BUG] Regression on behavior of EPOLLET | EPOLLIN for AF_UNIX sockets in 3.2
On 01/27/2012 09:53 PM, Eric Dumazet wrote:
> 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);
>
For the record, I just confirmed this to be the case.
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