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

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