linux-kernel - Re: [RT PATCH 3/3] hrtimer: Prevent using uninitialized spin_lock in hrtimer_grab_expiry

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Date:   Wed, 21 Aug 2019 16:02:54 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Julien Grall <julien.grall@....com>
cc:     linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        maz@...nel.org, bigeasy@...utronix.de, rostedt@...dmis.org
Subject: Re: [RT PATCH 3/3] hrtimer: Prevent using uninitialized spin_lock
 in hrtimer_grab_expiry_lock()

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