From: Thomas Gleixner <tglx@linutronix.de>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Julien Grall <julien.grall@arm.com>,
linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org,
maz@kernel.org, rostedt@goodmis.org
Subject: Re: [RT PATCH 1/3] hrtimer: Use READ_ONCE to access timer->base in hrimer_grab_expiry_lock()
Date: Wed, 21 Aug 2019 15:50:33 +0200 (CEST) [thread overview]
Message-ID: <alpine.DEB.2.21.1908211549040.2223@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20190821134437.efc3cs55o7uatrpj@linutronix.de>
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?
next prev parent reply other threads:[~2019-08-21 13:50 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-21 9:24 [RT PATCH 0/3] hrtimer: RT fixes for hrtimer_grab_expiry_lock() Julien Grall
2019-08-21 9:24 ` [RT PATCH 1/3] hrtimer: Use READ_ONCE to access timer->base in hrimer_grab_expiry_lock() Julien Grall
2019-08-21 13:44 ` Sebastian Andrzej Siewior
2019-08-21 13:50 ` Thomas Gleixner [this message]
2019-08-21 13:59 ` Sebastian Andrzej Siewior
2019-09-26 21:47 ` Eric Dumazet
2019-08-21 9:24 ` [RT PATCH 2/3] hrtimer: Don't grab the expiry lock for non-soft hrtimer Julien Grall
2019-08-21 13:49 ` Sebastian Andrzej Siewior
2019-08-21 9:24 ` [RT PATCH 3/3] hrtimer: Prevent using uninitialized spin_lock in hrtimer_grab_expiry_lock() Julien Grall
2019-08-21 14:02 ` Thomas Gleixner
2019-08-22 10:59 ` Julien Grall
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