linux-kernel - [PATCH] workaround minor lockdep bug triggered by mm_take_all

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Date:	Mon, 4 Aug 2008 19:57:30 +0200
From:	Andrea Arcangeli <andrea@...ranet.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Dave Jones <davej@...hat.com>, Roland Dreier <rdreier@...co.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Miller <davem@...emloft.net>, jeremy@...p.org,
	hugh@...itas.com, mingo@...e.hu, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] workaround minor lockdep bug triggered by mm_take_all_locks

From: Andrea Arcangeli <andrea@...ranet.com>

Lockdep can't recognize if spinlocks are at a different address. So
using trylock in a loop is one way to avoid lockdep to generate false
positives. After lockdep will be fixed this change can and should be
reverted.

Signed-off-by: Andrea Arcangeli <andrea@...ranet.com>
---

Resubmit because I didn't update the subject and so I improved the
comments too. This is mostly to show it's a bug in lockdep and it can
be trivially worked around without having to fix lockdep for real, any
superior solution to this hack is more than welcome and recommended.

diff -r 3469dce61df1 mm/mmap.c
--- a/mm/mmap.c	Tue Jul 29 20:01:28 2008 +0200
+++ b/mm/mmap.c	Mon Aug 04 19:54:27 2008 +0200
@@ -2279,8 +2279,12 @@ static void vm_lock_anon_vma(struct anon
 		/*
 		 * The LSB of head.next can't change from under us
 		 * because we hold the mm_all_locks_mutex.
+		 *
+		 * spin_lock would confuse lockdep who doesn't notice
+		 * the 'anon_vma' always changing address.
 		 */
-		spin_lock(&anon_vma->lock);
+		while (!spin_trylock(&anon_vma->lock))
+			cpu_relax();
 		/*
 		 * We can safely modify head.next after taking the
 		 * anon_vma->lock. If some other vma in this mm shares
@@ -2310,7 +2314,12 @@ static void vm_lock_mapping(struct addre
 		 */
 		if (test_and_set_bit(AS_MM_ALL_LOCKS, &mapping->flags))
 			BUG();
-		spin_lock(&mapping->i_mmap_lock);
+		/*
+		 * spin_lock would confuse lockdep who doesn't notice
+		 * the 'mapping' always changing address.
+		 */
+		while (!spin_trylock(&mapping->i_mmap_lock))
+			cpu_relax();
 	}
 }
 
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