Message-ID: <554A351D.8060700@linux.vnet.ibm.com>
Date: Wed, 06 May 2015 21:07:01 +0530
From: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
Organization: IBM
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To: Tahsin Erdogan <tahsin@google.com>
CC: tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com, x86@kernel.org,
        peterz@infradead.org, Waiman.Long@hp.com, borntraeger@de.ibm.com,
        oleg@redhat.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] x86/spinlocks: Fix regression in spinlock contention
 detection
References: <1430799331-20445-1-git-send-email-tahsin@google.com>
In-Reply-To: <1430799331-20445-1-git-send-email-tahsin@google.com>
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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@google.com>
> ---

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


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