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