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From: Thomas Gleixner <tglx@linutronix.de>
To: Julien Grall <julien.grall@arm.com>
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: Wed, 21 Aug 2019 16:02:54 +0200 (CEST)
Message-ID: <alpine.DEB.2.21.1908211557420.2223@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20190821092409.13225-4-julien.grall@arm.com>

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).

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.

Thanks,

	tglx

  reply index

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
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 [this message]
2019-08-22 10:59     ` Julien Grall

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