linux-kernel - Re: sem_lock() vs qspinlocks

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Message-ID: <20160520104500.GL2527@techsingularity.net>
Date:	Fri, 20 May 2016 11:45:00 +0100
From:	Mel Gorman <mgorman@...hsingularity.net>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Davidlohr Bueso <dave@...olabs.net>, manfred@...orfullife.com,
	Waiman.Long@....com, torvalds@...ux-foundation.org,
	ggherdovich@...e.com, linux-kernel@...r.kernel.org
Subject: Re: sem_lock() vs qspinlocks

On Fri, May 20, 2016 at 12:09:01PM +0200, Ingo Molnar wrote:
> 
> * Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > On Fri, May 20, 2016 at 10:30:08AM +0200, Peter Zijlstra wrote:
> > > On Thu, May 19, 2016 at 10:39:26PM -0700, Davidlohr Bueso wrote:
> > > > Specifically
> > > > for the 'cascade_cond' and 'cascade_flock' programs, which exhibit hangs in libc's
> > > > semop() blocked waiting for zero.
> > > 
> > > OK; so I've been running:
> > > 
> > > while :; do
> > >   bin/cascade_cond -E -C 200 -L -S -W -T 200  -I 2000000 ;
> > >   bin/cascade_flock -E -C 200 -L -S -W -P 200 -I 5000000 ;
> > > done
> > > 
> > > for a few minutes now and its not stuck and my machine didn't splat.
> > > 
> > > Am I not doing it right?
> > 
> > Hooray, it went *bang*..
> 
> I suspect a required step was to post about failure to reproduce!
> 

It is known that the bug is both intermittent and not all machines can
reproduce the problem. If it fails to reproduce, it's not necessarily a
methodology error and can simply be a function of luck.

-- 
Mel Gorman
SUSE Labs