Hi. As first mentioned on misc[1], I've identified a deadlock in libc when a process forks, the children are multi-threaded, and they set one or more atfork callbacks. The diff below causes ATFORK_UNLOCK() to release the lock even when the process isn't multi-threaded. This avoids the deadlock. With this patch applied, the test case I have for this issue succeeds and there are no new failures during a full 'make regress'.
Threading is outside my area of expertise so I've no idea if the fix proposed here is appropriate. I'm happy to take or test feedback. The diff is below and a clean copy is here: https://www.packetmischief.ca/files/patches/atfork_on_fork.diff. .joel [1] https://marc.info/?l=openbsd-misc&m=166926508819790&w=2 Index: lib/libc/include/thread_private.h =================================================================== RCS file: /data/cvs-mirror/OpenBSD/src/lib/libc/include/thread_private.h,v retrieving revision 1.36 diff -p -u -r1.36 thread_private.h --- lib/libc/include/thread_private.h 6 Jan 2021 19:54:17 -0000 1.36 +++ lib/libc/include/thread_private.h 8 Dec 2022 04:28:45 -0000 @@ -228,7 +228,7 @@ __END_HIDDEN_DECLS } while (0) #define _ATFORK_UNLOCK() \ do { \ - if (__isthreaded) \ + if (_thread_cb.tc_atfork_unlock != NULL) \ _thread_cb.tc_atfork_unlock(); \ } while (0) Index: regress/lib/libpthread/pthread_atfork_on_fork/Makefile =================================================================== RCS file: regress/lib/libpthread/pthread_atfork_on_fork/Makefile diff -N regress/lib/libpthread/pthread_atfork_on_fork/Makefile --- /dev/null 1 Jan 1970 00:00:00 -0000 +++ regress/lib/libpthread/pthread_atfork_on_fork/Makefile 7 Dec 2022 04:38:39 -0000 @@ -0,0 +1,9 @@ +# $OpenBSD$ + +PROG= pthread_atfork_on_fork + +REGRESS_TARGETS= timeout +timeout: + timeout 10s ./${PROG} + +.include <bsd.regress.mk> Index: regress/lib/libpthread/pthread_atfork_on_fork/pthread_atfork_on_fork.c =================================================================== RCS file: regress/lib/libpthread/pthread_atfork_on_fork/pthread_atfork_on_fork.c diff -N regress/lib/libpthread/pthread_atfork_on_fork/pthread_atfork_on_fork.c --- /dev/null 1 Jan 1970 00:00:00 -0000 +++ regress/lib/libpthread/pthread_atfork_on_fork/pthread_atfork_on_fork.c 7 Dec 2022 04:59:10 -0000 @@ -0,0 +1,94 @@ +/* $OpenBSD$ */ + +/* + * Copyright (c) 2022 Joel Knight <j...@openbsd.org> + * + * Permission to use, copy, modify, and distribute this software for any + * purpose with or without fee is hereby granted, provided that the above + * copyright notice and this permission notice appear in all copies. + * + * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES + * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF + * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR + * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES + * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN + * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF + * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. + */ + +/* + * This test exercises atfork lock/unlock through multiple generations of + * forked child processes where each child also becomes multi-threaded. + */ + +#include <pthread.h> +#include <stdio.h> +#include <stdlib.h> +#include <unistd.h> + +#include <sys/wait.h> + +#define NUMBER_OF_GENERATIONS 4 +#define NUMBER_OF_TEST_THREADS 2 + +#define SAY(...) do { \ + fprintf(stderr, "pid %5i ", getpid()); \ + fprintf(stderr, __VA_ARGS__); \ +} while (0) + +void +prepare(void) +{ + /* Do nothing */ +} + +void * +thread(void *arg) +{ + return (NULL); +} + +int +test(int fork_level) +{ + pid_t proc_pid; + size_t thread_index; + pthread_t threads[NUMBER_OF_TEST_THREADS]; + + proc_pid = fork(); + fork_level = fork_level - 1; + + if (proc_pid == 0) { + SAY("generation %i\n", fork_level); + pthread_atfork(prepare, NULL, NULL); + + for (thread_index = 0; thread_index < NUMBER_OF_TEST_THREADS; thread_index++) { + pthread_create(&threads[thread_index], NULL, thread, NULL); + } + + for (thread_index = 0; thread_index < NUMBER_OF_TEST_THREADS; thread_index++) { + pthread_join(threads[thread_index], NULL); + } + + if (fork_level > 0) { + test(fork_level); + } + + SAY("exiting\n"); + exit(0); + } + else { + SAY("parent waiting on child %i\n", proc_pid); + waitpid(proc_pid, 0, 0); + } + + return (0); +} + +int +main(int argc, char *argv[]) +{ + test(NUMBER_OF_GENERATIONS); + + return (0); +}
