Re: lockdep trace from posix timers

[Ming Lei]

On Sat, Jul 28, 2012 at 12:20 AM, Dave Jones <da...@redhat.com> wrote:
> On Tue, Jul 24, 2012 at 04:36:13PM -0400, Dave Jones wrote:
>  > Linus tree as of 5fecc9d8f59e765c2a48379dd7c6f5cf88c7d75a
>  >
>  >      Dave
>  >
>  > ======================================================
>  > [ INFO: HARDIRQ-safe -> HARDIRQ-unsafe lock order detected ]
>  > 3.5.0+ #122 Not tainted
>  > ------------------------------------------------------
>  > trinity-child2/5327 [HC0[0]:SC0[0]:HE0:SE1] is trying to acquire:
>  > blocked:  (tasklist_lock){.+.+..}, instance: ffffffff81c05098, at: 
> [<ffffffff8109762b>] posix_cpu_timer_del+0x2b/0xe0
>  >
>  > and this task is already holding:
>  > blocked:  (&(&new_timer->it_lock)->rlock){-.-...}, instance: 
> ffff880143bce170, at: [<ffffffff81093d49>] __lock_timer+0x89/0x1f0
>  > which would create a new lock dependency:
>  >  (&(&new_timer->it_lock)->rlock){-.-...} -> (tasklist_lock){.+.+..}
>  >
>  > but this new dependency connects a HARDIRQ-irq-safe lock:
>  >  (&(&new_timer->it_lock)->rlock){-.-...}
>  > ... which became HARDIRQ-irq-safe at:
>
> Shall I start bisecting this ? I can trigger it very easily, but if you
> can give me a set of commits to narrow down, it'll speed up the bisection.

It should a real possible deadlock, could you test the below patch to
see if it can fix the warning?

--
diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index 125cb67..29f6a8e 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -420,7 +420,7 @@ static int posix_cpu_timer_del(struct k_itimer *timer)
        int ret = 0;

        if (likely(p != NULL)) {
-               read_lock(&tasklist_lock);
+               /* tasklist_lock held already in timer_delete */
                if (unlikely(p->sighand == NULL)) {
                        /*
                         * We raced with the reaping of the task.
@@ -435,7 +435,6 @@ static int posix_cpu_timer_del(struct k_itimer *timer)
                                list_del(&timer->it.cpu.entry);
                        spin_unlock(&p->sighand->siglock);
                }
-               read_unlock(&tasklist_lock);

                if (!ret)
                        put_task_struct(p);
diff --git a/kernel/posix-timers.c b/kernel/posix-timers.c
index 69185ae..222d24c 100644
--- a/kernel/posix-timers.c
+++ b/kernel/posix-timers.c
@@ -884,15 +884,31 @@ SYSCALL_DEFINE1(timer_delete, timer_t, timer_id)
        struct k_itimer *timer;
        unsigned long flags;

+       /*
+        * hold tasklist_lock to protect tsk->sighand which might be
+        * accessed inside ->timer_del. It should be held before
+        * timer->it_lock to avoid the below deadlock:
+        *      CPU0                    CPU1
+        *      lock(tasklist_lock)
+        *                              timer_delete()
+        *                                      lock(timer->it_lock)
+        *                                      lock(tasklist_lock)
+        *      <timer interrupt>
+        *              lock(timer->it_lock)
+        */
+       read_lock(&tasklist_lock);
 retry_delete:
        timer = lock_timer(timer_id, &flags);
-       if (!timer)
+       if (!timer) {
+               read_unlock(&tasklist_lock);
                return -EINVAL;
+       }

        if (timer_delete_hook(timer) == TIMER_RETRY) {
                unlock_timer(timer, flags);
                goto retry_delete;
        }
+       read_unlock(&tasklist_lock);

        spin_lock(&current->sighand->siglock);
        list_del(&timer->list);


Thanks,
-- 
Ming Lei
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