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Date: Thu, 30 Nov 2000 22:27:25 +0100
From: Andrea Arcangeli <andrea@suse.de>
To: V Ganesh <ganesh@veritas.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: beware of add_waitqueue/waitqueue_active
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In-Reply-To: <200011301332.TAA00277@vxindia.veritas.com>; from ganesh@veritas.com on Thu, Nov 30, 2000 at 07:02:56PM +0530
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On Thu, Nov 30, 2000 at 07:02:56PM +0530, V Ganesh wrote:
> 3. add_wait_queue adds this process to the waitqueue. but all the writes
>    are in write-buffers and have not gone down to cache/memory yet.
> 4. PageLocked() finds that the page is locked.

Right.

> [..] speculative execution
> of PageLocked() even before add_wait_queue returns [..]

Right.

Both could happen because of the new spin_unlock implementation that doesn't
take anymore the lock on the bus:

	#define spin_unlock_string \
		"movb $1,%0"

With the previous `lock ; btrl' 4) couldn't happen with pending
writes on the write buffer because spin_unlock was a full barrier too...

Alpha was safe because it has to imply an mb() in spin_unlock (but
sparc64 was hurted too for example).

As you say the problematic construct is not used anymore into 2.4.0-test12-pre2
in filemap.c so such deadlock can't happen anymore but it's been useful that
you pointing out the problem, thanks.

Andrea
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