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From: Julien Grall <julien.grall@arm.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org,
	maz@kernel.org, bigeasy@linutronix.de, rostedt@goodmis.org
Subject: Re: [RT PATCH 3/3] hrtimer: Prevent using uninitialized spin_lock in hrtimer_grab_expiry_lock()
Date: Thu, 22 Aug 2019 11:59:10 +0100
Message-ID: <6f637e70-9a7b-47fd-08b0-82b6494d3ae1@arm.com> (raw)
In-Reply-To: <alpine.DEB.2.21.1908211557420.2223@nanos.tec.linutronix.de>

Hi Thomas,

Thank you for the review.

On 21/08/2019 15:02, Thomas Gleixner wrote:
> On Wed, 21 Aug 2019, Julien Grall wrote:
> 
>> migration_base is used as a placeholder when an hrtimer is switching
>> between base (see switch_hrtimer_timer_base). It is possible
>> theoritically possible to have timer->base equal to migration_base.
>>
>> Even if it is a placeholder, it would pass all the current check in
>> hrtimer_grab_expiry_lock() leading to use softirq_expiry_lock
>> uninitialized.
>>
>> This is can be prevented by checking whether the base is equal to
>> the placeholder (i.e. migration_base).
> 
> That's a lame argument. The point is that it does not make sense to do that
> on migration base, but not for the reason you are giving (uninitialized
> lock).

Fair point, I will update the commit message.

> 
> If base == migration_base then there is no point to lock soft_expiry_lock
> simply because the timer is not executing the callback in soft irq context
> and the whole lock/unlock dance can be avoided.
> 
> But, yes. Good catch.

Do you want me to resend the series or can I just provide an update to the 
commit message here?

Cheers,

-- 
Julien Grall

  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
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 [this message]
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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