linux-kernel - Re: [PATCH] hrtimer: Add a missing bracket and hide `migration

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Date:   Wed, 4 Sep 2019 16:39:18 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
cc:     linux-kernel@...r.kernel.org, Julien Grall <julien.grall@....com>
Subject: Re: [PATCH] hrtimer: Add a missing bracket and hide `migration_base'
 on !SMP

On Wed, 4 Sep 2019, Sebastian Andrzej Siewior wrote:

> Commit
>    68b2c8c1e4210 ("hrtimer: Don't take expiry_lock when timer is currently migrated")

That is the same information as in the fixes tag. So you can say something
like this:

The recent change to avoid taking the expiry lock when a timer is currently
migrated missed .....

> missed to add a bracket at the end of the if statement leading to
> compile errors.
> Since that commit the variable `migration_base' is always used but only
> available on SMP configuration thus leading to another compile error.
> The changelog says
>   "The timer base and base->cpu_base cannot be NULL in the code path"
> 
> so it is safe to limit this check to SMP configurations only.
> 
> Add the missing bracket to the if statement and hide `migration_base'
> behind CONFIG_SMP bars.
> 
> Fixes: 68b2c8c1e4210 ("hrtimer: Don't take expiry_lock when timer is currently migrated")
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>  kernel/time/hrtimer.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
> index f5a1a5e16216c..bc84c74ae5b96 100644
> --- a/kernel/time/hrtimer.c
> +++ b/kernel/time/hrtimer.c
> @@ -1216,12 +1216,16 @@ void hrtimer_cancel_wait_running(const struct hrtimer *timer)
>  {
>  	/* Lockless read. Prevent the compiler from reloading it below */
>  	struct hrtimer_clock_base *base = READ_ONCE(timer->base);
> +	bool is_migration_base = false;
>  
>  	/*
>  	 * Just relax if the timer expires in hard interrupt context or if
>  	 * it is currently on the migration base.
>  	 */
> -	if (!timer->is_soft || base == &migration_base)
> +#ifdef CONFIG_SMP
> +	is_migration_base = base == &migration_base;
> +#endif

That's beyond ugly.

What's wrong with:

        if (!timer->is_soft || is_migration_base(base))

and have two helpers in the relevant ifdeffed section?

Thanks,

	tglx