linux-kernel - Re: Potential data race in dput and __d

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Message-ID: <20141120180503.GN7996@ZenIV.linux.org.uk>
Date:	Thu, 20 Nov 2014 18:05:03 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Andrey Konovalov <andreyknvl@...gle.com>
Cc:	Ingo Molnar <mingo@...nel.org>, Will Deacon <will.deacon@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Davidlohr Bueso <davidlohr@...com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Kostya Serebryany <kcc@...gle.com>, ktsan@...glegroups.com
Subject: Re: Potential data race in dput and __d_lookup

On Thu, Nov 20, 2014 at 09:20:14PM +0400, Andrey Konovalov wrote:

> It seems that one thread increments 'dentry->d_lockref.count', while
> other does 'lockref_put_or_lock(&dentry->d_lockref)'  without any
> synchronization.
> 
> Could you confirm if this is a real race?

I can not.  Increment in __d_lookup() is under ->d_lockref.lock (aka
->d_lock) and lockref_put_or_lock() is atomic wrt that.
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