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

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Date:	Mon, 09 Mar 2015 10:37:10 -0700
From:	Jason Low <jason.low2@...com>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	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>, jason.low2@...com
Subject: Re: [PATCH] locking/rwsem: Fix lock optimistic spinning when owner
 is not running

On Sat, 2015-03-07 at 13:17 -0500, Sasha Levin wrote:
> 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.

Great, thanks for confirming this.

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