Re: [PATCH -v6 04/13] futex,rt_mutex: Provide futex specific rt_mutex API

From: Darren Hart <dvhart@infradead.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: tglx@linutronix.de, mingo@kernel.org, juri.lelli@arm.com,
	rostedt@goodmis.org, xlpang@redhat.com, bigeasy@linutronix.de,
	linux-kernel@vger.kernel.org, mathieu.desnoyers@efficios.com,
	jdesfossez@efficios.com, bristot@redhat.com
Subject: Re: [PATCH -v6 04/13] futex,rt_mutex: Provide futex specific rt_mutex API
Date: Thu, 6 Apr 2017 10:08:03 -0700	[thread overview]
Message-ID: <20170406170803.GB7030@fury> (raw)
In-Reply-To: <20170406121728.h3lj2fg4bbst4663@hirez.programming.kicks-ass.net>

On Thu, Apr 06, 2017 at 02:17:28PM +0200, Peter Zijlstra wrote:
> On Wed, Apr 05, 2017 at 08:02:17AM -0700, Darren Hart wrote:
> > > @@ -1364,20 +1364,18 @@ static int wake_futex_pi(u32 __user *uad
> > >  	pi_state->owner = new_owner;
> > >  	raw_spin_unlock(&new_owner->pi_lock);
> > >  
> > >  	/*
> > > +	 * We've updated the uservalue, this unlock cannot fail.
> > 
> > It isn't clear to me what I should understand from this new comment. How does
> > the value of the uval affect whether or not the pi_state->pi_mutex can be
> > unlocked or not? Or are you noting that we've set FUTEX_WAITIERS so any valid
> > userspace operations will be forced intot he kernel and can't race with us since
> > we hold the hb->lock? With futexes, I think it's important that we be very
> > explicit in our comment blocks.
> 
> The critical point is that once you've modified uval we must not fail;
> there is no way to undo things thereafter.

Aha, "must not", OK. I interpretted "cannot" as "is incapable of failing". So
let's use something like that for the comment:

/*
 * We updated the user value and are committed to completing the unlock, we must
 * not fail.
 */

Wow... English. I tried a few versions, but cannot, may not, etc. all have
doublt meanings. :-)

-- 
Darren Hart
VMware Open Source Technology Center