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

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Date:   Wed, 21 Aug 2019 15:59:31 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Thomas Gleixner <tglx@...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 2019-08-21 15:50:33 [+0200], Thomas Gleixner wrote:
> 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?
so read_once() it is then.

Sebastian