linux-kernel - Re: [PATCH 1/3] hrtimer: kill HRTIMER_STATE_MIGRATE, fix the race with hrtimer_is

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Message-ID: <20150608151333.GF23138@redhat.com>
Date:	Mon, 8 Jun 2015 17:13:33 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	umgwanakikbuti@...il.com, mingo@...e.hu, ktkhai@...allels.com,
	rostedt@...dmis.org, tglx@...utronix.de, juri.lelli@...il.com,
	pang.xunlei@...aro.org, wanpeng.li@...ux.intel.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] hrtimer: kill HRTIMER_STATE_MIGRATE, fix the race
	with hrtimer_is_queued()

Sorry, for double/wrong posting. Please ignore this one, I've resent
it in reply to 0/3.

On 06/08, Oleg Nesterov wrote:
>
> migrate_hrtimer_list() sets HRTIMER_STATE_MIGRATE to avoid the
> race with hrtimer_active(), but this can't avoid the same race
> with hrtimer_is_queued().
> 
> Kill HRTIMER_STATE_MIGRATE, we can simply use STATE_ENQUEUED.
> ---
>  include/linux/hrtimer.h |    6 +-----
>  kernel/time/hrtimer.c   |    7 ++-----
>  2 files changed, 3 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/hrtimer.h b/include/linux/hrtimer.h
> index 05f6df1..44810bc 100644
> --- a/include/linux/hrtimer.h
> +++ b/include/linux/hrtimer.h
> @@ -70,17 +70,13 @@ enum hrtimer_restart {
>   * the handling of the timer.
>   *
>   * The HRTIMER_STATE_ENQUEUED bit is always or'ed to the current state
> - * to preserve the HRTIMER_STATE_CALLBACK in the above scenario. This
> - * also affects HRTIMER_STATE_MIGRATE where the preservation is not
> - * necessary. HRTIMER_STATE_MIGRATE is cleared after the timer is
> - * enqueued on the new cpu.
> + * to preserve the HRTIMER_STATE_CALLBACK in the above scenario.
>   *
>   * All state transitions are protected by cpu_base->lock.
>   */
>  #define HRTIMER_STATE_INACTIVE	0x00
>  #define HRTIMER_STATE_ENQUEUED	0x01
>  #define HRTIMER_STATE_CALLBACK	0x02
> -#define HRTIMER_STATE_MIGRATE	0x04
>  
>  /**
>   * struct hrtimer - the basic hrtimer structure
> diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
> index 76d4bd9..005fd44 100644
> --- a/kernel/time/hrtimer.c
> +++ b/kernel/time/hrtimer.c
> @@ -1640,11 +1640,11 @@ static void migrate_hrtimer_list(struct hrtimer_clock_base *old_base,
>  		debug_deactivate(timer);
>  
>  		/*
> -		 * Mark it as STATE_MIGRATE not INACTIVE otherwise the
> +		 * Mark it as ENQUEUED not INACTIVE otherwise the
>  		 * timer could be seen as !active and just vanish away
>  		 * under us on another CPU
>  		 */
> -		__remove_hrtimer(timer, old_base, HRTIMER_STATE_MIGRATE, 0);
> +		__remove_hrtimer(timer, old_base, HRTIMER_STATE_ENQUEUED, 0);
>  		timer->base = new_base;
>  		/*
>  		 * Enqueue the timers on the new cpu. This does not
> @@ -1655,9 +1655,6 @@ static void migrate_hrtimer_list(struct hrtimer_clock_base *old_base,
>  		 * event device.
>  		 */
>  		enqueue_hrtimer(timer, new_base);
> -
> -		/* Clear the migration state bit */
> -		timer->state &= ~HRTIMER_STATE_MIGRATE;
>  	}
>  }
>  
> -- 
> 1.5.5.1
> 

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