linux-kernel - Re: [PATCH] x86/spinlocks: Fix regression in spinlock contention detection

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Message-ID: <554A351D.8060700@linux.vnet.ibm.com>
Date:	Wed, 06 May 2015 21:07:01 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Tahsin Erdogan <tahsin@...gle.com>
CC:	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	x86@...nel.org, peterz@...radead.org, Waiman.Long@...com,
	borntraeger@...ibm.com, oleg@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/spinlocks: Fix regression in spinlock contention
 detection

On 05/05/2015 09:45 AM, Tahsin Erdogan wrote:
> A spinlock is regarded as contended when there is at least one waiter.
> Currently, the code that checks whether there are any waiters rely on
> tail value being greater than head. However, this is not true if tail
> reaches the max value and wraps back to zero, so arch_spin_is_contended()
> incorrectly returns 0 (not contended) when tail is smaller than head.
>
> The original code (before regression) handled this case by casting the
> (tail - head) to an unsigned value. This change simply restores that
> behavior.
>
> Fixes: d6abfdb20223 ("x86/spinlocks/paravirt: Fix memory corruption on
> unlock")
> Signed-off-by: Tahsin Erdogan <tahsin@...gle.com>
> ---

Tahsin,
Perhaps we need to CC stable (3.19) too..?


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