linux-kernel - Re: [PATCH -v5 12/14] futex,rt_mutex: Restructure rt_mutex_finish_proxy

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Message-ID: <alpine.DEB.2.20.1703071859040.4299@nanos>
Date:   Tue, 7 Mar 2017 18:59:57 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     mingo@...nel.org, juri.lelli@....com, rostedt@...dmis.org,
        xlpang@...hat.com, bigeasy@...utronix.de,
        linux-kernel@...r.kernel.org, mathieu.desnoyers@...icios.com,
        jdesfossez@...icios.com, bristot@...hat.com, dvhart@...radead.org
Subject: Re: [PATCH -v5 12/14] futex,rt_mutex: Restructure
 rt_mutex_finish_proxy_lock()

On Tue, 7 Mar 2017, Peter Zijlstra wrote:

> On Tue, Mar 07, 2017 at 03:18:46PM +0100, Thomas Gleixner wrote:
> > On Sat, 4 Mar 2017, Peter Zijlstra wrote:
> > > +/**
> > > + * rt_mutex_cleanup_proxy_lock() - Cleanup failed lock acquisition
> > > + * @lock:		the rt_mutex we were woken on
> > > + * @waiter:		the pre-initialized rt_mutex_waiter
> > > + *
> > > + * Clean up the failed lock acquisition as per rt_mutex_wait_proxy_lock().
> > > + *
> > > + * Returns:
> > > + *  true  - did the cleanup, we done.
> > > + *  false - we acquired the lock after rt_mutex_wait_proxy_lock() returned,
> > > + *          caller should disregards its return value.
> > 
> > Hmm. How would that happen? Magic owner assignement to a non waiter? The
> > callsite only calls here in the failed case.
> 
> Ah, but until the remove_waiter() below, we _still_ are a waiter, and
> thus can get assigned ownership.
> 
> > > + *
> > > + * Special API call for PI-futex support
> > > + */
> > > +bool rt_mutex_cleanup_proxy_lock(struct rt_mutex *lock,
> > > +				 struct rt_mutex_waiter *waiter)
> > > +{
> > > +	bool cleanup = false;
> > > +
> > > +	raw_spin_lock_irq(&lock->wait_lock);
> > > +	/*
> > > +	 * If we acquired the lock, no cleanup required.
> > > +	 */
> > > +	if (rt_mutex_owner(lock) != current) {
> > > +		remove_waiter(lock, waiter);
> 
> See, up till this point, we still a waiter and any unlock can see us
> being one.

Hmm, true. So the comments should explain that

Thanks,

	tglx