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From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Thomas Gleixner <tglx@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 15:59:31 +0200
Message-ID: <20190821135931.x6s2b2cwvrxgvoyi@linutronix.de> (raw)
In-Reply-To: <alpine.DEB.2.21.1908211549040.2223@nanos.tec.linutronix.de>

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

  reply index

Thread overview: 16+ 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
2019-08-21 13:59       ` Sebastian Andrzej Siewior [this message]
2019-09-26 21:47       ` Eric Dumazet
2019-08-23  2:12   ` [tip: timers/core] hrtimer: Protect lockless access to timer->base tip-bot2 for Julien Grall
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
2019-08-23  2:12   ` [tip: timers/core] hrtimer: Don't take expiry_lock when timer is currently migrated tip-bot2 for Julien Grall
2019-09-04 14:15     ` [PATCH] hrtimer: Add a missing bracket and hide `migration_base' on !SMP Sebastian Andrzej Siewior
2019-09-04 14:39       ` Thomas Gleixner
2019-09-04 14:55         ` [PATCH v2] " Sebastian Andrzej Siewior

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