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Message-ID: <1428609577.24035.3.camel@stgolabs.net>
Subject: Re: [PATCH 2/2] locking/rwsem: Use a return variable in
 rwsem_spin_on_owner()
From: Davidlohr Bueso <dave@stgolabs.net>
To: Jason Low <jason.low2@hp.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Ingo Molnar <mingo@kernel.org>, Peter Zijlstra <peterz@infradead.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Aswin Chandramouleeswaran <aswin@hp.com>,
        LKML <linux-kernel@vger.kernel.org>
Date: Thu, 09 Apr 2015 12:59:37 -0700
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On Thu, 2015-04-09 at 12:43 -0700, Jason Low wrote:
> So that looks more similar to how the original code was where the
> rcu_read_lock() and rcu_read_unlock() was done inside the owner_running
> helper function (though without the CONFIG_DEBUG_PAGEALLOC), before
> commit 307bf9803f25 ("sched: Simplify mutex_spin_on_owner()") modified
> it to be done outside the loop.

I think this is why Linus was mentioning the CONFIG_PREEMPT_RCU case as
well, so taking and releasing the read lock in those cases in the loop
would actually hurt more.

Thanks,
Davidlohr