linux-kernel - Re: [PATCH 1/4] sched/wakeup: Strengthen current_save_and_set_rtlock_wait

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Date:   Fri, 10 Sep 2021 12:07:40 -0400
From:   Waiman Long <llong@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>
Cc:     tglx@...utronix.de, boqun.feng@...il.com,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Mike Galbraith <efault@....de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH 1/4] sched/wakeup: Strengthen
 current_save_and_set_rtlock_wait_state()

On 9/10/21 10:01 AM, Peter Zijlstra wrote:
> On Fri, Sep 10, 2021 at 03:17:04PM +0200, Peter Zijlstra wrote:
>> On Fri, Sep 10, 2021 at 01:57:26PM +0100, Will Deacon wrote:
>>> On Thu, Sep 09, 2021 at 04:27:46PM +0200, Peter Zijlstra wrote:
>>>> Moo yes, so the earlier changelog I wrote was something like:
>>>>
>>>> 	current_save_and_set_rtlock_wait_state();
>>>> 	for (;;) {
>>>> 		if (try_lock())
>>>> 			break;
>>>>
>>>> 		raw_spin_unlock_irq(&lock->wait_lock);
>>>> 		if (!cond)
>>>> 			schedule();
>>>> 		raw_spin_lock_irq(&lock->wait_lock);
>>>>
>>>> 		set_current_state(TASK_RTLOCK_WAIT);
>>>> 	}
>>>> 	current_restore_rtlock_saved_state();
>>>>
>>>> which is more what the code looks like before these patches, and in that
>>>> case the @cond load can be lifted before __state.
>>> Ah, so that makes more sense, thanks. I can't see how the try_lock() could
>>> be reordered though, as it's going to have to do an atomic rmw.
>> OK, lemme go update the Changelog and make it __flags for bigeasy :-)
> The patch now reads:
>
> ---
> Subject: sched/wakeup: Strengthen current_save_and_set_rtlock_wait_state()
> From: Peter Zijlstra <peterz@...radead.org>
> Date: Thu, 09 Sep 2021 12:59:16 +0200
>
> While looking at current_save_and_set_rtlock_wait_state() I'm thinking
> it really ought to use smp_store_mb(), because using it for a more
> traditional wait loop like:
>
> 	current_save_and_set_rtlock_wait_state();
> 	for (;;) {
> 		if (cond)
> 			schedule();
>
> 		set_current_state(TASK_RTLOCK_WAIT);
> 	}
> 	current_restore_rtlock_saved_state();
>
> is actually broken, since the cond load could be re-ordered against
> the state store, which could lead to a missed wakeup -> BAD (tm).
>
> While there, make them consistent with the IRQ usage in
> set_special_state().
>
> Fixes: 5f220be21418 ("sched/wakeup: Prepare for RT sleeping spin/rwlocks")
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Link: https://lkml.kernel.org/r/20210909110203.767330253@infradead.org
> ---
>   include/linux/sched.h |   21 ++++++++++++---------
>   1 file changed, 12 insertions(+), 9 deletions(-)
>
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -218,7 +218,7 @@ struct task_group;
>    */
>   #define set_special_state(state_value)					\
>   	do {								\
> -		unsigned long flags; /* may shadow */			\
> +		unsigned long __flags; /* may shadow */			\

Do you still need the "may shadow" comment?

Cheers,
Longman