I'm finding ppoll() unexpectedly stuck when waiting for POLLIN on a local TCP socket. The isolated code below can reproduces the issue after many minutes (<1 hour). It might be easier to reproduce on a busy system while disk I/O is happening.
This may also be related to an epoll-related issue reported by Andreas Voellmy: http://thread.gmane.org/gmane.linux.kernel/1408782/ My example involves a 3 thread data flow between two pairs of (4) sockets: send_loop -> recv_loop(recv_send) -> recv_loop(recv_only) pair_a[1] -> (pair_a[0] -> pair_b[1]) -> pair_b[0] At least 3.7 and 3.7.1 are affected. I have tcp_low_latency=1 set, I will try 0 later The last progress message I got was after receiving 2942052597760 bytes on fd=7 (out of 64-bit ULONG_MAX / 2) strace: 3644 sendto(4, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 16384, 0, NULL, 0 <unfinished ...> 3643 sendto(6, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 16384, 0, NULL, 0 <unfinished ...> 3642 ppoll([{fd=7, events=POLLIN}], 1, NULL, NULL, 8 <unfinished ...> 3641 futex(0x7f23ed8129d0, FUTEX_WAIT, 3644, NULL <unfinished ...> The first and last lines of the strace are expected: + 3644 sendto(4) is blocked because 3643 is blocked on sendto(fd=6) and not able to call recv(). + 3641 is the main thread calling pthread_join What is unexpected is the tid=3643 and tid=3642 interaction. As confirmed by lsof below, fd=6 is sending to wake up fd=7, but ppoll(fd=7) seems to not be waking up. lsof: toosleepy 3641 ew 4u IPv4 12405 0t0 TCP localhost:55904->localhost:33249 (ESTABLISHED) toosleepy 3641 ew 5u IPv4 12406 0t0 TCP localhost:33249->localhost:55904 (ESTABLISHED) toosleepy 3641 ew 6u IPv4 12408 0t0 TCP localhost:48777->localhost:33348 (ESTABLISHED) toosleepy 3641 ew 7u IPv4 12409 0t0 TCP localhost:33348->localhost:48777 (ESTABLISHED) System info: Linux 3.7.1 x86_64 SMP PREEMPT AMD Phenom(tm) II X4 945 Processor (4 cores) Nothing interesting in dmesg, iptables rules are empty. I have not yet been able to reproduce the issue using UNIX sockets, only TCP, but you can run: ./toosleepy unix ...to test with UNIX sockets intead of TCP. The following code is also available via git://bogomips.org/toosleepy gcc -o toosleepy -O2 -Wall -lpthread toosleepy.c -------------------------------- 8< ------------------------------------ #define _GNU_SOURCE #include <poll.h> #include <sys/ioctl.h> #include <pthread.h> #include <sys/types.h> #include <sys/socket.h> #include <arpa/inet.h> #include <netinet/tcp.h> #include <stdint.h> #include <stdio.h> #include <stdlib.h> #include <errno.h> #include <string.h> #include <unistd.h> #include <fcntl.h> #include <assert.h> #include <limits.h> struct receiver { int rfd; int sfd; }; /* blocking sender */ static void * send_loop(void *fdp) { int fd = *(int *)fdp; char buf[16384]; ssize_t s; size_t sent = 0; size_t max = (size_t)ULONG_MAX / 2; while (sent < max) { s = send(fd, buf, sizeof(buf), 0); if (s > 0) sent += s; if (s == -1) assert(errno == EINTR); } dprintf(2, "%d done sending: %zu\n", fd, sent); close(fd); return NULL; } /* non-blocking receiver, using ppoll */ static void * recv_loop(void *p) { const struct receiver *rcvr = p; char buf[16384]; nfds_t nfds = 1; struct pollfd fds; int rc; ssize_t r, s; size_t received = 0; size_t sent = 0; for (;;) { r = recv(rcvr->rfd, buf, sizeof(buf), 0); if (r == 0) { break; } else if (r == -1) { assert(errno == EAGAIN); fds.fd = rcvr->rfd; fds.events = POLLIN; errno = 0; rc = ppoll(&fds, nfds, NULL, NULL); assert(rc == 1); } else { assert(r > 0); received += r; if (rcvr->sfd >= 0) { s = send(rcvr->sfd, buf, sizeof(buf), 0); if (s > 0) sent += s; if (s == -1) assert(errno == EINTR); } else { /* just burn some cycles */ write(-1, buf, sizeof(buf)); } } if ((received % (sizeof(buf) * sizeof(buf) * 16) == 0)) dprintf(2, " %d progress: %zu\n", rcvr->rfd, received); } dprintf(2, "%d got: %zu\n", rcvr->rfd, received); if (rcvr->sfd >= 0) { dprintf(2, "%d sent: %zu\n", rcvr->sfd, sent); close(rcvr->sfd); } return NULL; } static void tcp_socketpair(int sv[2], int accept_flags) { struct sockaddr_in addr; socklen_t addrlen = sizeof(addr); int l = socket(PF_INET, SOCK_STREAM, 0); int c = socket(PF_INET, SOCK_STREAM, 0); int a; addr.sin_family = AF_INET; addr.sin_addr.s_addr = INADDR_ANY; addr.sin_port = 0; assert(0 == bind(l, (struct sockaddr*)&addr, addrlen)); assert(0 == listen(l, 1024)); assert(0 == getsockname(l, (struct sockaddr *)&addr, &addrlen)); assert(0 == connect(c, (struct sockaddr *)&addr, addrlen)); a = accept4(l, NULL, NULL, accept_flags); assert(a >= 0); close(l); sv[0] = a; sv[1] = c; } int main(int argc, char *argv[]) { int pair_a[2]; int pair_b[2]; pthread_t s, rs, r; struct receiver recv_only; struct receiver recv_send; if (argc == 2 && strcmp(argv[1], "unix") == 0) { int val; assert(0 == socketpair(AF_UNIX, SOCK_STREAM, 0, pair_a)); assert(0 == socketpair(AF_UNIX, SOCK_STREAM, 0, pair_b)); /* only make the receiver non-blocking */ val = 1; assert(0 == ioctl(pair_a[0], FIONBIO, &val)); val = 1; assert(0 == ioctl(pair_b[0], FIONBIO, &val)); } else { tcp_socketpair(pair_a, SOCK_NONBLOCK); tcp_socketpair(pair_b, SOCK_NONBLOCK); } recv_send.rfd = pair_a[0]; recv_send.sfd = pair_b[1]; recv_only.rfd = pair_b[0]; recv_only.sfd = -1; /* * data flow: * send_loop -> recv_loop(recv_send) -> recv_loop(recv_only) * pair_a[1] -> (pair_a[0] -> pair_b[1]) -> pair_b[0] */ assert(0 == pthread_create(&r, NULL, recv_loop, &recv_only)); assert(0 == pthread_create(&rs, NULL, recv_loop, &recv_send)); assert(0 == pthread_create(&s, NULL, send_loop, &pair_a[1])); assert(0 == pthread_join(s, NULL)); assert(0 == pthread_join(rs, NULL)); assert(0 == pthread_join(r, NULL)); return 0; } -------------------------------- 8< ------------------------------------ Any help/suggestions/test patches would be greatly appreciated. Thanks for reading! -- Eric Wong -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
