linux-kernel - Re: [PATCH] locking/rwsem: Fix lock optimistic spinning when owner is not running

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Message-ID: <54FB40BE.1070309@oracle.com>
Date:	Sat, 07 Mar 2015 13:17:34 -0500
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Jason Low <jason.low2@...com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
CC:	Davidlohr Bueso <dave@...olabs.net>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Michel Lespinasse <walken@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...emonkey.org.uk>,
	Ming Lei <ming.lei@...onical.com>
Subject: Re: [PATCH] locking/rwsem: Fix lock optimistic spinning when owner
 is not running

On 03/07/2015 02:45 AM, Jason Low wrote:
> Fixes tip commit b3fd4f03ca0b (locking/rwsem: Avoid deceiving lock spinners).
> 
> Ming reported soft lockups occurring when running xfstest due to
> commit b3fd4f03ca0b.
> 
> When doing optimistic spinning in rwsem, threads should stop spinning when
> the lock owner is not running. While a thread is spinning on owner, if
> the owner reschedules, owner->on_cpu returns false and we stop spinning.
> 
> However, commit b3fd4f03ca0b essentially caused the check to get ignored
> because when we break out of the spin loop due to !on_cpu, we continue
> spinning if sem->owner != NULL.
> 
> This patch fixes this by making sure we stop spinning if the owner is not
> running. Furthermore, just like with mutexes, refactor the code such that
> we don't have separate checks for owner_running(). This makes it more
> straightforward in terms of why we exit the spin on owner loop and we
> would also avoid needing to "guess" why we broke out of the loop to make
> this more readable.

That seems to solve the hangs I'm seeing as well.


Thanks,
Sasha
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