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Subject: deadlock in epoll (found by lockdep)
From: Arjan van de Ven <arjan@infradead.org>
To: davidel@xmailserver.org, linux-kernel@vger.kernel.org
Cc: akpm@osdl.org, mingo@elte.hu
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Date: Thu, 01 Jun 2006 10:45:37 +0200
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Hi,

in ep_poll() (fs/eventpoll.c) the code does

       write_lock_irqsave(&ep->lock, flags);

        res = 0;
        if (list_empty(&ep->rdllist)) {
                /*
                 * We don't have any available event to return to the caller.
                 * We need to sleep here, and we will be wake up by
                 * ep_poll_callback() when events will become available.
                 */
                init_waitqueue_entry(&wait, current);
                add_wait_queue(&ep->wq, &wait);

eg we first take ep->lock and then call add_wait_queue which takes 
         spin_lock_irqsave(&q->lock, flags);
for obvious reasons.
this would mean that ep->lock would be the outer lock, and q->lock the
inner lock.


HOWEVER, __wake_up does this:
void fastcall __wake_up(wait_queue_head_t *q, unsigned int mode,
                        int nr_exclusive, void *key)
{
        unsigned long flags;

        spin_lock_irqsave(&q->lock, flags);
        __wake_up_common(q, mode, nr_exclusive, 0, key);
        spin_unlock_irqrestore(&q->lock, flags);
}

where __wake_up_common calls into ep_poll_callback, which in term does
        write_lock_irqsave(&ep->lock, flags);
as one of the first things.

... which implies that q->lock is the outer lock, and ep->lock is the
inner lock.... 

can you explain which order is right, and if/why this is not an AB-BA
deadlock??

Greetings,
   Arjan van de Ven