Messages in this thread Patch in this message | | | Date | Sat, 24 Sep 2005 17:42:19 +0400 | From | Oleg Nesterov <> | Subject | [PATCH] fix exit_itimers() vs posix_timer_event() AB-BA deadlock |
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CPU_0 CPU_1
release_task: posix_timer_fn: write_lock(tasklist); spin_lock(timer->it_lock);
exit_timers: send_sigqueue: spin_lock(timer->it_lock) read_lock(tasklist);
Actually, it is a bit worse. If posix timer starts between tasklist locking and del_timer_sync() call this deadlock will happen because of TIMER_RETRY logic.
With this patch posix_timer_event() detects the exiting thread group and aborts the sending of signal (taking tasklist_lock). To simplify the code tasklist locking moved from send_{,group_}sigqueue to the posix_timer_event().
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
--- 2.6.14-rc2/kernel/posix-timers.c~1_DLOCK 2005-09-17 18:57:30.000000000 +0400 +++ 2.6.14-rc2/kernel/posix-timers.c 2005-09-24 18:03:05.000000000 +0400 @@ -411,6 +411,10 @@ exit: int posix_timer_event(struct k_itimer *timr,int si_private) { + struct task_struct *leader, *zombie; + int (*send_actor)(int, struct sigqueue *, struct task_struct *); + int ret; + memset(&timr->sigq->info, 0, sizeof(siginfo_t)); timr->sigq->info.si_sys_private = si_private; /* @@ -428,22 +432,40 @@ int posix_timer_event(struct k_itimer *t timr->sigq->info.si_tid = timr->it_id; timr->sigq->info.si_value = timr->it_sigev_value; - if (timr->it_sigev_notify & SIGEV_THREAD_ID) { - struct task_struct *leader; - int ret = send_sigqueue(timr->it_sigev_signo, timr->sigq, - timr->it_process); - - if (likely(ret >= 0)) - return ret; - - timr->it_sigev_notify = SIGEV_SIGNAL; - leader = timr->it_process->group_leader; - put_task_struct(timr->it_process); - timr->it_process = leader; - } - - return send_group_sigqueue(timr->it_sigev_signo, timr->sigq, - timr->it_process); + /* + * We are locking ->it_lock + tasklist_lock backwards + * from release_task()->exit_itimers(), beware deadlock. + */ + leader = timr->it_process->group_leader; + while (unlikely(!read_trylock(&tasklist_lock))) { + if (leader->flags & PF_EXITING) { + smp_rmb(); + if (thread_group_empty(leader)) + return 0; + } + cpu_relax(); + } + + zombie = NULL; + send_actor = send_group_sigqueue; + if (timr->it_sigev_notify & SIGEV_THREAD_ID) { + if (unlikely(timr->it_process->flags & PF_EXITING)) { + zombie = timr->it_process; + timr->it_process = leader; + timr->it_sigev_notify = SIGEV_SIGNAL; + } else + send_actor = send_sigqueue; + } + + ret = send_actor(timr->it_sigev_signo, timr->sigq, + timr->it_process); + + read_unlock(&tasklist_lock); + + if (unlikely(zombie != NULL)) + put_task_struct(zombie); + + return ret; } EXPORT_SYMBOL_GPL(posix_timer_event); --- 2.6.14-rc2/kernel/signal.c~1_DLOCK 2005-09-17 18:57:30.000000000 +0400 +++ 2.6.14-rc2/kernel/signal.c 2005-09-24 18:01:11.000000000 +0400 @@ -1367,22 +1367,14 @@ send_sigqueue(int sig, struct sigqueue * int ret = 0; BUG_ON(!(q->flags & SIGQUEUE_PREALLOC)); - read_lock(&tasklist_lock); - - if (unlikely(p->flags & PF_EXITING)) { - ret = -1; - goto out_err; - } - spin_lock_irqsave(&p->sighand->siglock, flags); if (unlikely(!list_empty(&q->list))) { + BUG_ON(q->info.si_code != SI_TIMER); /* * If an SI_TIMER entry is already queue just increment * the overrun count. */ - if (q->info.si_code != SI_TIMER) - BUG(); q->info.si_overrun++; goto out; } @@ -1400,9 +1392,6 @@ send_sigqueue(int sig, struct sigqueue * out: spin_unlock_irqrestore(&p->sighand->siglock, flags); -out_err: - read_unlock(&tasklist_lock); - return ret; } @@ -1413,7 +1402,6 @@ send_group_sigqueue(int sig, struct sigq int ret = 0; BUG_ON(!(q->flags & SIGQUEUE_PREALLOC)); - read_lock(&tasklist_lock); spin_lock_irqsave(&p->sighand->siglock, flags); handle_stop_signal(sig, p); @@ -1424,13 +1412,12 @@ send_group_sigqueue(int sig, struct sigq } if (unlikely(!list_empty(&q->list))) { + BUG_ON(q->info.si_code != SI_TIMER); /* * If an SI_TIMER entry is already queue just increment * the overrun count. Other uses should not try to * send the signal multiple times. */ - if (q->info.si_code != SI_TIMER) - BUG(); q->info.si_overrun++; goto out; } @@ -1447,8 +1434,7 @@ send_group_sigqueue(int sig, struct sigq __group_complete_signal(sig, p); out: spin_unlock_irqrestore(&p->sighand->siglock, flags); - read_unlock(&tasklist_lock); - return(ret); + return ret; } /* - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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