linux-kernel - Re: [PATCH 2/5] futex: Check for pi futex

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Date:	Sat, 23 Nov 2013 21:19:05 -0800
From:	Davidlohr Bueso <davidlohr@...com>
To:	Darren Hart <dvhart@...ux.intel.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	peterz@...radead.org, tglx@...utronix.de, efault@....de,
	jeffm@...e.com, torvalds@...ux-foundation.org, scott.norton@...com,
	tom.vaden@...com, aswin@...com, Waiman.Long@...com,
	jason.low2@...com
Subject: Re: [PATCH 2/5] futex: Check for pi futex_q only once

On Fri, 2013-11-22 at 22:33 -0800, Darren Hart wrote:
> On Fri, 2013-11-22 at 16:56 -0800, Davidlohr Bueso wrote:
> > All wake_futex() callers already verify that the we are not dealing with
> > a pi futex_q, so we can remove the redundant WARN() check, as this is never
> > triggered anyway.
> > 
> > Cc: Ingo Molnar <mingo@...nel.org>
> > Cc: Darren Hart <dvhart@...ux.intel.com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Mike Galbraith <efault@....de>
> > Cc: Jeff Mahoney <jeffm@...e.com>
> > Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> > Cc: Scott Norton <scott.norton@...com>
> > Cc: Tom Vaden <tom.vaden@...com>
> > Cc: Aswin Chandramouleeswaran <aswin@...com>
> > Cc: Waiman Long <Waiman.Long@...com>
> > Cc: Jason Low <jason.low2@...com>
> > Signed-off-by: Davidlohr Bueso <davidlohr@...com>
> > ---
> >  kernel/futex.c | 3 ---
> >  1 file changed, 3 deletions(-)
> > 
> > diff --git a/kernel/futex.c b/kernel/futex.c
> > index e6ffe73..0768c68 100644
> > --- a/kernel/futex.c
> > +++ b/kernel/futex.c
> > @@ -844,9 +844,6 @@ static void wake_futex(struct futex_q *q)
> >  {
> >  	struct task_struct *p = q->task;
> >  
> > -	if (WARN(q->pi_state || q->rt_waiter, "refusing to wake PI futex\n"))
> > -		return;
> > -
> 
> This was added deliberately after adding said checks to the callers...
> admittedly after a very long debug session I didn't ever want to repeat.
> Sometimes warnings are added to make sure we caught everything and later
> removed.... sometimes they are added to make sure nothing new ever
> breaks this again. Since the failure scenario is non-obvious, unless
> this is causing some significant performance issues for you, I'd prefer
> this stays.
> 
> See commit aa10990e028cac3d5e255711fb9fb47e00700e35 for details.

I think a strong comment in wake_futex() warning about pi futex_qs would
do nowadays, but fair enough, I was kind of expecting this reply.

Thanks,
Davidlohr


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