Oleg,
On Mon, 2005-08-22 at 12:52 +0400, Oleg Nesterov wrote:
> Thomas Gleixner wrote:
> I think yes, and this is closely related to your and Paul
> "Use RCU to protect tasklist for unicast signals" patches.
>
> We don't need RCU here, but we do need this hypothetical
> lock_task_sighand() (and __exit_sighand from __exit_signal
> change).
>
> We still need tasklist_lock in send_group_queue for
> __group_complete_signal though. I hope Paul will solve
> this, it is needed for group_send_info() anyway.
>
> But for the near future I don't see simple and non-intrusive
> fix for this deadlock. I am waiting for George to confirm
> that the bug exists and we are not crazy :)
It exists. It triggers on preempt-RT and I can trigger it on vanilla SMP
by waiting for the timer expiry in release_task() before the
__exit_signal() call. That's reasonable, as it can happen that way by
chance too. It requires that the timer expires on a different CPU, but I
don't see a reason why this should not happen.
The patch below solves it for me.
tglx
diff -uprN --exclude-from=/usr/local/bin/diffit.exclude linux-2.6.13-rc6/kernel/exit.c linux-2.6.13-rc6.signal/kernel/exit.c
--- linux-2.6.13-rc6/kernel/exit.c 2005-08-13 12:25:58.000000000 +0200
+++ linux-2.6.13-rc6.signal/kernel/exit.c 2005-08-22 11:19:02.000000000 +0200
@@ -71,7 +71,6 @@ repeat:
__ptrace_unlink(p);
BUG_ON(!list_empty(&p->ptrace_list) || !list_empty(&p->ptrace_children));
__exit_signal(p);
- __exit_sighand(p);
/*
* Note that the fastpath in sys_times depends on __exit_signal having
* updated the counters before a task is removed from the tasklist of
diff -uprN --exclude-from=/usr/local/bin/diffit.exclude linux-2.6.13-rc6/kernel/fork.c linux-2.6.13-rc6.signal/kernel/fork.c
--- linux-2.6.13-rc6/kernel/fork.c 2005-08-13 12:25:58.000000000 +0200
+++ linux-2.6.13-rc6.signal/kernel/fork.c 2005-08-22 11:35:05.000000000 +0200
@@ -1132,7 +1132,8 @@ bad_fork_cleanup_mm:
bad_fork_cleanup_signal:
exit_signal(p);
bad_fork_cleanup_sighand:
- exit_sighand(p);
+ if (p->sighand)
+ exit_sighand(p);
bad_fork_cleanup_fs:
exit_fs(p); /* blocking */
bad_fork_cleanup_files:
diff -uprN --exclude-from=/usr/local/bin/diffit.exclude linux-2.6.13-rc6/kernel/posix-timers.c linux-2.6.13-rc6.signal/kernel/posix-timers.c
--- linux-2.6.13-rc6/kernel/posix-timers.c 2005-08-13 12:25:58.000000000 +0200
+++ linux-2.6.13-rc6.signal/kernel/posix-timers.c 2005-08-21 23:19:42.000000000 +0200
@@ -427,21 +427,23 @@ int posix_timer_event(struct k_itimer *t
timr->sigq->info.si_code = SI_TIMER;
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) {
- if (unlikely(timr->it_process->flags & PF_EXITING)) {
- timr->it_sigev_notify = SIGEV_SIGNAL;
- put_task_struct(timr->it_process);
- timr->it_process = timr->it_process->group_leader;
- goto group;
- }
- return send_sigqueue(timr->it_sigev_signo, timr->sigq,
- timr->it_process);
- }
- else {
- group:
- return send_group_sigqueue(timr->it_sigev_signo, timr->sigq,
- timr->it_process);
+ 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);
}
EXPORT_SYMBOL_GPL(posix_timer_event);
@@ -499,7 +501,8 @@ static void posix_timer_fn(unsigned long
remove_from_abslist(timr);
}
- if (posix_timer_event(timr, si_private))
+ /* Do not rearm the timer, when we are exiting */
+ if (posix_timer_event(timr, si_private) > 0)
/*
* signal was not sent because of sig_ignor
* we will not get a call back to restart it AND
diff -uprN --exclude-from=/usr/local/bin/diffit.exclude linux-2.6.13-rc6/kernel/signal.c linux-2.6.13-rc6.signal/kernel/signal.c
--- linux-2.6.13-rc6/kernel/signal.c 2005-08-13 12:25:58.000000000 +0200
+++ linux-2.6.13-rc6.signal/kernel/signal.c 2005-08-22 11:51:08.000000000 +0200
@@ -395,6 +395,7 @@ void __exit_signal(struct task_struct *t
}
clear_tsk_thread_flag(tsk,TIF_SIGPENDING);
flush_sigqueue(&tsk->pending);
+ __exit_sighand(tsk);
if (sig) {
/*
* We are cleaning up the signal_struct here. We delayed
@@ -1380,16 +1381,20 @@ send_sigqueue(int sig, struct sigqueue *
unsigned long flags;
int ret = 0;
- /*
- * We need the tasklist lock even for the specific
- * thread case (when we don't need to follow the group
- * lists) in order to avoid races with "p->sighand"
- * going away or changing from under us.
- */
BUG_ON(!(q->flags & SIGQUEUE_PREALLOC));
- read_lock(&tasklist_lock);
+
+ while(!read_trylock(&tasklist_lock)) {
+ if (p->flags & PF_EXITING)
+ return -1;
+ cpu_relax();
+ }
+ if (unlikely(p->flags & PF_EXITING)) {
+ ret = -1;
+ goto out_err;
+ }
+
spin_lock_irqsave(&p->sighand->siglock, flags);
-
+
if (unlikely(!list_empty(&q->list))) {
/*
* If an SI_TIMER entry is already queue just increment
@@ -1399,7 +1404,7 @@ send_sigqueue(int sig, struct sigqueue *
BUG();
q->info.si_overrun++;
goto out;
- }
+ }
/* Short-circuit ignored signals. */
if (sig_ignored(p, sig)) {
ret = 1;
@@ -1414,8 +1419,10 @@ send_sigqueue(int sig, struct sigqueue *
out:
spin_unlock_irqrestore(&p->sighand->siglock, flags);
+out_err:
read_unlock(&tasklist_lock);
- return(ret);
+
+ return ret;
}
int
@@ -1425,7 +1432,16 @@ send_group_sigqueue(int sig, struct sigq
int ret = 0;
BUG_ON(!(q->flags & SIGQUEUE_PREALLOC));
- read_lock(&tasklist_lock);
+
+ while(!read_trylock(&tasklist_lock)) {
+ if (!p->sighand)
+ return -1;
+ cpu_relax();
+ }
+ if (unlikely(!p->sighand)) {
+ ret = -1;
+ goto out_err;
+ }
spin_lock_irqsave(&p->sighand->siglock, flags);
handle_stop_signal(sig, p);
@@ -1459,8 +1475,9 @@ send_group_sigqueue(int sig, struct sigq
__group_complete_signal(sig, p);
out:
spin_unlock_irqrestore(&p->sighand->siglock, flags);
+out_err:
read_unlock(&tasklist_lock);
- return(ret);
+ return ret;
}
/*
-
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