linux-kernel - Re: [PATCH 001 of 9] md: Fix deadlock in md/raid1 and md/raid10 when handling a read error.

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Date:	Thu, 6 Mar 2008 11:51:34 +0100
From:	Andre Noll <maan@...temlinux.org>
To:	Neil Brown <neilb@...e.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org,
	"K.Tanaka" <k-tanaka@...jp.nec.com>
Subject: Re: [PATCH 001 of 9] md: Fix deadlock in md/raid1 and md/raid10 when handling a read error.

On 14:29, Neil Brown wrote:
> > Are you worried about another CPU setting conf->pending_bio_list.head
> > to != NULL after the if statement? If that's an issue I think also
> > the original patch is problematic because the same might happen after
> > the final spin_unlock_irq() but but before flush_pending_writes()
> > returns zero.
> 
> No.  I'm worried that another CPU might set
> conf->pending_bio_list.head *before* the if statement, but it isn't
> seen by this CPU because of the lack of memory barriers.  The spinlock
> ensures that the memory state is consistent.

But is that enough to avoid the deadlock? I think the following
scenario would be possible with the code in the original patch:

	// suppose conf->pending_bio_list.head==NULL ATM

	CPU0:
	int rv = 0;
	spin_lock_irq(&conf->device_lock);
	if (conf->pending_bio_list.head) // false
	spin_unlock_irq(&conf->device_lock);

	CPU1:
	conf->pending_bio_list.head = something;

	CPU0:
	return rv; // zero

Andre
-- 
The only person who always got his work done by Friday was Robinson Crusoe

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