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SubjectRe: [bug] af_unix: Reading from a stream socket may lock the concurrent poll() call
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Le lundi 21 novembre 2011 à 00:19 +0400, Alexey Moiseytsev a écrit :
> Hello,
>
> The following program shows how the poll() call hangs on a non-empty
> stream socket.
>
> #include <sys/types.h>
> #include <sys/socket.h>
> #include <pthread.h>
> #include <stdio.h>
> #include <unistd.h>
> #include <poll.h>
>
> int sockets[2];
>
> int poll_socket(void) {
> struct pollfd pfd = {
> .fd = sockets[1],
> .events = POLLIN
> };
> return poll(&pfd, 1, -1);
> }
>
>
> /* observer routine doesn't modify amount of data available in the
> socket buffer */
> void* observer(void* arg) {
> char buffer;
> for (int j = 0; j < 2000; j++) {
> recv(sockets[1], &buffer, sizeof(buffer), MSG_PEEK);
> sched_yield();
> }
> return NULL;
> }
>
> int main(void) {
> if (socketpair(PF_UNIX, SOCK_STREAM, 0, sockets) == -1)
> return 1;
> int rc, data[250] = {0};
> if ((rc = send(sockets[0], &data, sizeof(data), MSG_DONTWAIT)) <= 0)
> return 2;
> poll_socket();
> /* If the first poll_socket() call didn't hang then the following
> message will be printed */
> fprintf(stderr, "%d bytes available in input buffer\n", rc);
> pthread_t observer_thread;
> pthread_create(&observer_thread, NULL, observer, NULL);
> for (int j = 0; j < 20000; j++) {
> /* If the first poll_socket() call didn't hang then all the following
> calls should do the same */
> poll_socket();
> }
> fprintf(stderr, "Well done\n");
> pthread_join(observer_thread, NULL);
> close(sockets[0]);
> close(sockets[1]);
> return 0;
> }
>
>
> Expected output: two lines or nothing (in case of error).
> Observed output: only the first line (and the process never exits).
>
> So the first poll() said that there is some data available in the
> socket. And one of the following poll() said that there is no data
> available in the socket. But this is false because the observer thread
> didn't actually consume any data from then socket.
>
> I assume that this bug can be eliminated by adding
> sk->sk_data_ready(...) call right after each call to
> skb_queue_head(..) in the unix_stream_recvmsg(...) routine
> (net/unix/af_unix.c)
>
> Other info:
> $ uname -srmo
> Linux 2.6.32-5-amd64 x86_64 GNU/Linux
>

Hi Alexy

I believe you found a bug and your suggested fix should be just fine.

(Or maybe testing in unix_poll() that at least one thread is currently
handling one skb from sk->receive_queue)

Could you submit an official patch on top of current Linus tree or do
you prefer us to take care of this ?



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