linux-kernel - Re: [PATCH 2/2] rwsem: do optimistic spinning for writer lock acquisition

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Message-ID: <1372109438.22432.75.camel@schen9-DESK>
Date:	Mon, 24 Jun 2013 14:30:38 -0700
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Davidlohr Bueso <davidlohr.bueso@...com>,
	Alex Shi <alex.shi@...el.com>,
	Michel Lespinasse <walken@...gle.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Matthew R Wilcox <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH 2/2] rwsem: do optimistic spinning for writer lock
 acquisition

On Mon, 2013-06-24 at 16:48 -0400, Peter Hurley wrote:

> 
> Also, I haven't given a lot of thought to if preemption must be disabled
> before calling rwsem_can_spin_on_owner(). If so, wouldn't you just drop
> rwsem_can_spin_on_owner() (because the conditions tested in the loop are
> equivalent)?
> 

Not totally equivalent.  If we drop the call to rwsem_can_spin_on_owner,
we will spin when readers are holding the lock (owner is null).  
Right now we only allow writers to spin when other writers are 
holding the lock by adding the rwsem_can_spin_on_owner check.  
Letting spinning on readers held lock is tricky as
we could have a reader that sleeps and if we don't detect the case.  We
could spin for too long.

Thanks.

Tim


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