linux-kernel - Re: [PATCH v2] locking/rtmutex: Fix incorrect spinning condition

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Message-ID: <87k0g3ar7z.ffs@tglx>
Date:   Fri, 17 Dec 2021 21:53:04 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Zqiang <qiang1.zhang@...el.com>, peterz@...radead.org,
        mingo@...hat.com, will@...nel.org, longman@...hat.com
Cc:     linux-kernel@...r.kernel.org, qiang1.zhang@...el.com
Subject: Re: [PATCH v2] locking/rtmutex: Fix incorrect spinning condition

Zqiang,

On Fri, Dec 17 2021 at 15:42, Zqiang wrote:
> When the lock owner is on CPU and not need resched, the current waiter
> need to be checked, if it not longer top the waiter, stop spinning.
>
> Fixes: c3123c431447 ("locking/rtmutex: Dont dereference waiter lockless")
> Signed-off-by: Zqiang <qiang1.zhang@...el.com>
> ---
>  v1->v2:
>  Modify description information.
>
>  kernel/locking/rtmutex.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
> index 0c1f2e3f019a..8555c4efe97c 100644
> --- a/kernel/locking/rtmutex.c
> +++ b/kernel/locking/rtmutex.c
> @@ -1383,7 +1383,7 @@ static bool rtmutex_spin_on_owner(struct rt_mutex_base *lock,
>  		 *  - the VCPU on which owner runs is preempted
>  		 */
>  		if (!owner_on_cpu(owner) || need_resched() ||
> -		    rt_mutex_waiter_is_top_waiter(lock, waiter)) {
> +		    !rt_mutex_waiter_is_top_waiter(lock, waiter)) {
>  			res = false;
>  			break;

good catch!

Though this does not apply because the condition is incomplete. You
somehow dropped this from the condition:

                   vcpu_is_preempted(task_cpu(owner))) 

Please make always sure that your patches apply against Linus tree
before sending them out.

Thanks,

        tglx