linux-kernel - Re: [RT PATCH 1/3] hrtimer: Use READ_ONCE to access timer->base in hrimer_grab_expiry

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Message-ID: <alpine.DEB.2.21.1908211549040.2223@nanos.tec.linutronix.de>
Date:   Wed, 21 Aug 2019 15:50:33 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
cc:     Julien Grall <julien.grall@....com>,
        linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        maz@...nel.org, rostedt@...dmis.org
Subject: Re: [RT PATCH 1/3] hrtimer: Use READ_ONCE to access timer->base in
 hrimer_grab_expiry_lock()

On Wed, 21 Aug 2019, Sebastian Andrzej Siewior wrote:

> On 2019-08-21 10:24:07 [+0100], Julien Grall wrote:
> > The update to timer->base is protected by the base->cpu_base->lock().
> > However, hrtimer_grab_expirty_lock() does not access it with the lock.
> > 
> > So it would theorically be possible to have timer->base changed under
> > our feet. We need to prevent the compiler to refetch timer->base so the
> > check and the access is performed on the same base.
> 
> It is not a problem if the timer's bases changes. We get here because we
> want to help the timer to complete its callback.
> The base can only change if the timer gets re-armed on another CPU which
> means is completed callback. In every case we can cancel the timer on
> the next iteration.

It _IS_ a problem when the base changes and the compiler reloads

   CPU0	  	       	   	CPU1
   base = timer->base;

   lock(base->....);
				switch base

   reload
	base = timer->base;

   unlock(base->....);

See?