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: <20190821134437.efc3cs55o7uatrpj@linutronix.de>
Date:   Wed, 21 Aug 2019 15:44:37 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Julien Grall <julien.grall@....com>
Cc:     linux-rt-users@...r.kernel.org, tglx@...utronix.de,
        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 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.

Sebastian