Date: Wed, 4 Feb 2015 13:06:21 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Jason Low <jason.low2@hp.com>, Davidlohr Bueso <dave@stgolabs.net>,
        Ingo Molnar <mingo@kernel.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Michel Lespinasse <walken@google.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 4/5] locking/rwsem: Avoid deceiving lock spinners
Message-ID: <20150204120621.GH23123@twins.programming.kicks-ass.net>
References: <1422609267-15102-1-git-send-email-dave@stgolabs.net>
 <1422609267-15102-5-git-send-email-dave@stgolabs.net>
 <1422669098.9530.33.camel@schen9-desk2.jf.intel.com>
 <1422671289.28351.1.camel@stgolabs.net>
 <1422983812.9530.43.camel@schen9-desk2.jf.intel.com>
 <1422986041.2368.3.camel@j-VirtualBox>
 <1422992616.9530.78.camel@schen9-desk2.jf.intel.com>
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On Tue, Feb 03, 2015 at 11:43:36AM -0800, Tim Chen wrote:
> That's true.  We cannot have the lock grabbed by a new write 
> contender as any new writer contender of the lock will be 
> queued by the OSQ logic. Only the
> thread doing the optimistic spin is attempting write lock.  
> In other word, switching of write owner of the rwsem to a new
> owner cannot happen.  Either write owner stay as the original one, or
> we don't have a write owner.  So using test of write owner
> switching as an indicator of congestion is incorrect.
> 
> If my reasoning above is sound, then the check 
> 
> +       if (READ_ONCE(sem->owner))
> +               return true; /* new owner, continue spinning */
> +
> 
> is unnecessary and can be removed, as we cannot have a 
> new write owner of the rwsem, other than the thread
> doing optimistic spinning.

I have read the rest of the thread; but the one thing that I didn't see
is trylocks, trylocks can always come in an steal things regardless of
the OSQ stuff.
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