Re: [PATCH] sigqueue_free: fix the race with collect_signal()

[taoyue]

Oleg Nesterov wrote:

On 08/24, taoyue wrote:

Oleg Nesterov wrote:

--- t/kernel/signal.c~SQFREE    2007-08-22 20:06:31.000000000 +0400
+++ t/kernel/signal.c   2007-08-23 16:02:57.000000000 +0400
@@ -1297,20 +1297,19 @@ struct sigqueue *sigqueue_alloc(void)
void sigqueue_free(struct sigqueue *q)
{
        unsigned long flags;
+       spinlock_t *lock = &current->sighand->siglock;
+
        BUG_ON(!(q->flags & SIGQUEUE_PREALLOC));
        /*
         * If the signal is still pending remove it from the
-        * pending queue.
+        * pending queue. We must hold ->siglock while testing
+        * q->list to serialize with collect_signal().
         */
-       if (unlikely(!list_empty(&q->list))) {
-               spinlock_t *lock = &current->sighand->siglock;
-               read_lock(&tasklist_lock);
-               spin_lock_irqsave(lock, flags);
-               if (!list_empty(&q->list))
-                       list_del_init(&q->list);
-               spin_unlock_irqrestore(lock, flags);
-               read_unlock(&tasklist_lock);
-       }
+       spin_lock_irqsave(lock, flags);
+       if (!list_empty(&q->list))
+               list_del_init(&q->list);
+       spin_unlock_irqrestore(lock, flags);
+
        q->flags &= ~SIGQUEUE_PREALLOC;
        __sigqueue_free(q);
}

Applying previous patch???it seems likely that the __sigqueue_free() is also called twice.

collect_signal:                         sigqueue_free:

        list_del_init(&first->list);
                                       spin_lock_irqsave(lock, flags);

                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

                                       if (!list_empty(&q->list))
                                             list_del_init(&q->list);
                                       spin_unlock_irqrestore(lock, flags);
                                       q->flags &= ~SIGQUEUE_PREALLOC;

       __sigqueue_free(first);          __sigqueue_free(q);

collect_signal() is always called under ->siglock which is also taken by
sigqueue_free(), so this is not possible.

Basically, this patch is the same one-liner I sent you before

        http://marc.info/?l=linux-kernel&m=118772206603453&w=2

(Thanks for the additional testing and report, btw).

P.S. It would be nice to know if this patch solves the problems reported
by Jeremy, but his email is disabled.

Oleg.

I know, using current->sighand->siglock to prevent one sigqueue
is free twice. I want to know whether it is possible that the two
function is called in different thread. If that, the spin_lock is useless.

yue.tao
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