From: Aleksandr Sergeev <[email protected]> By the spec, fork() copies only the thread which executes it. So it may happen, what while one thread is doing a fork, another thread is holding `clone_lock` mutex (e.g. doing a `fork()` or `exit()`). So the child process is born with the mutex being held, and there are nobody to release it.
As the thread executing do_syscall() is not considered running, start_exclusive() does not protect us from the case. Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3226 Signed-off-by: Aleksandr Sergeev <[email protected]> Reviewed-by: Richard Henderson <[email protected]> Signed-off-by: Richard Henderson <[email protected]> Message-ID: <[email protected]> --- linux-user/user-internals.h | 2 ++ linux-user/main.c | 2 ++ linux-user/syscall.c | 14 ++++++++++++++ 3 files changed, 18 insertions(+) diff --git a/linux-user/user-internals.h b/linux-user/user-internals.h index 067c02bb93..24d35998f0 100644 --- a/linux-user/user-internals.h +++ b/linux-user/user-internals.h @@ -69,6 +69,8 @@ abi_long get_errno(abi_long ret); const char *target_strerror(int err); int get_osversion(void); void init_qemu_uname_release(void); +void clone_fork_start(void); +void clone_fork_end(bool child); void fork_start(void); void fork_end(pid_t pid); diff --git a/linux-user/main.c b/linux-user/main.c index db751c0757..c49d1e91d2 100644 --- a/linux-user/main.c +++ b/linux-user/main.c @@ -145,6 +145,7 @@ unsigned long guest_stack_size = TARGET_DEFAULT_STACK_SIZE; void fork_start(void) { start_exclusive(); + clone_fork_start(); mmap_fork_start(); cpu_list_lock(); qemu_plugin_user_prefork_lock(); @@ -174,6 +175,7 @@ void fork_end(pid_t pid) cpu_list_unlock(); } gdbserver_fork_end(thread_cpu, pid); + clone_fork_end(child); /* * qemu_init_cpu_list() reinitialized the child exclusive state, but we * also need to keep current_cpu consistent, so call end_exclusive() for diff --git a/linux-user/syscall.c b/linux-user/syscall.c index 3e6a56aa0f..d466d0e32f 100644 --- a/linux-user/syscall.c +++ b/linux-user/syscall.c @@ -6856,6 +6856,20 @@ static void *clone_func(void *arg) return NULL; } +void clone_fork_start(void) +{ + pthread_mutex_lock(&clone_lock); +} + +void clone_fork_end(bool child) +{ + if (child) { + pthread_mutex_init(&clone_lock, NULL); + } else { + pthread_mutex_unlock(&clone_lock); + } +} + /* do_fork() Must return host values and target errnos (unlike most do_*() functions). */ static int do_fork(CPUArchState *env, unsigned int flags, abi_ulong newsp, -- 2.43.0
