Date: Mon, 7 Jan 2008 20:22:39 +0300
From: Oleg Nesterov <oleg@tv-sign.ru>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Herbert Xu <herbert@gondor.apana.org.au>, Ingo Molnar <mingo@elte.hu>,
       "Rafael J. Wysocki" <rjw@sisk.pl>,
       Christian Kujau <lists@nerdbynature.de>, linux-kernel@vger.kernel.org,
       jfs-discussion@lists.sourceforge.net,
       Davide Libenzi <davidel@xmailserver.org>,
       Johannes Berg <johannes@sipsolutions.net>
Subject: Re: 2.6.24-rc6: possible recursive locking detected
Message-ID: <20080107172239.GA14880@tv-sign.ru>
References: <alpine.DEB.0.999999.0801032349190.4821@sheep.housecafe.de> <200801040006.47979.rjw@sisk.pl> <20080104083049.GC22803@elte.hu> <20080105071205.GA28936@gondor.apana.org.au> <1199552016.31975.41.camel@lappy> <1199552476.31975.45.camel@lappy>
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On 01/05, Peter Zijlstra wrote:
>
> Since EP_MAX_POLLWAKE_NESTS < MAX_LOCKDEP_SUBCLASSES we could perhaps do
> something like:
>
>   wake_up_nested(..., wake_nests);

I think this would be the most correct change. But I wonder if it is possible
to do something more generic (but otoh more stupid/hackish and less safe).

Consider this "just for illustration" patch,

--- t/kernel/lockdep.c	2007-11-09 12:57:31.000000000 +0300
+++ t/kernel/lockdep.c	2008-01-07 19:43:50.000000000 +0300
@@ -1266,10 +1266,13 @@ check_deadlock(struct task_struct *curr,
 	struct held_lock *prev;
 	int i;
 
-	for (i = 0; i < curr->lockdep_depth; i++) {
+	for (i = curr->lockdep_depth; --i >= 0; ) {
 		prev = curr->held_locks + i;
 		if (prev->class != next->class)
 			continue;
+
+		if (prev->trylock == -1)
+			return 2;
 		/*
 		 * Allow read-after-read recursion of the same
 		 * lock class (i.e. read_lock(lock)+read_lock(lock)):
-------------------------------------------------------------------------

Now,

	// trylock == -1
	#define	spin_mark_nested(l)	\
		lock_acquire(&(l)->dep_map, 0, -1, 0, 2, _THIS_IP_)
	#define	spin_unmark_nested(l)	\
		lock_release(&(l)->dep_map, 1, _THIS_IP_)

and ep_poll_safewake() can do:

	/* Do really wake up now */
	spin_mark_nested(&wq->lock);
	wake_up(wq);
	spin_unmark_nested(&wq->lock);

Possible?

Oleg.

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