linux-kernel - Re: [patch 1/4] timer: Use deferrable base independent of base::nohz

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Date:   Mon, 25 Dec 2017 17:24:21 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Ingo Molnar <mingo@...nel.org>, stable@...r.kernel.org,
        rt@...utronix.de
Subject: Re: [patch 1/4] timer: Use deferrable base independent of
 base::nohz_active

On Fri, Dec 22, 2017 at 03:51:12PM +0100, Thomas Gleixner wrote:
> From: Anna-Maria Gleixner <anna-maria@...utronix.de>
> 
> During boot and before base::nohz_active is set in the timer bases, deferrable
> timers are enqueued into the standard timer base. This works correctly as
> long as base::nohz_active is false.
> 
> Once it base::nohz_active is set and a timer which was enqueued before that
> is accessed the lock selector code choses the lock of the deferred
> base. This causes unlocked access to the standard base and in case the
> timer is removed it does not clear the pending flag in the standard base
> bitmap which causes get_next_timer_interrupt() to return bogus values.
> 
> To prevent that, the deferrable timers must be enqueued in the deferrable
> base, even when base::nohz_active is not set. Those deferrable timers also
> need to be expired unconditional.
> 
> Fixes: 500462a9de65 ("timers: Switch to a non-cascading wheel")
> Signed-off-by: Anna-Maria Gleixner <anna-maria@...utronix.de>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Cc: stable@...r.kernel.org
> Cc: rt@...utronix.de

Nice catch!

Reviewed-by: Frederic Weisbecker <frederic@...nel.org>

Thanks!