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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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