linux-kernel - Re: [PATCH] rcu: Fix opposite might_sleep() check in rcu_blocking_is

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Date:   Fri, 16 Dec 2022 17:03:45 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Zqiang <qiang1.zhang@...el.com>
Cc:     frederic@...nel.org, quic_neeraju@...cinc.com,
        joel@...lfernandes.org, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu: Fix opposite might_sleep() check in
 rcu_blocking_is_gp()

On Thu, Dec 15, 2022 at 11:57:55AM +0800, Zqiang wrote:
> Currently, if the system is in the RCU_SCHEDULER_INACTIVE state, invoke
> synchronize_rcu_*() will implies a grace period and return directly,
> so there is no sleep action due to waiting for a grace period to end,
> but this might_sleep() check is the opposite. therefore, this commit
> puts might_sleep() check in the correct palce.
> 
> Signed-off-by: Zqiang <qiang1.zhang@...el.com>

Queued for testing and review, thank you!

I was under the impression that might_sleep() did some lockdep-based
checking, but I am unable to find it.  If there really is such checking,
that would be a potential argument for leaving this code as it is.

But in the meantime, full speed ahead!  ;-)

						Thanx, Paul

> ---
>  kernel/rcu/tree.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index ee8a6a711719..65f3dd2fd3ae 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3379,9 +3379,10 @@ void __init kfree_rcu_scheduler_running(void)
>   */
>  static int rcu_blocking_is_gp(void)
>  {
> -	if (rcu_scheduler_active != RCU_SCHEDULER_INACTIVE)
> +	if (rcu_scheduler_active != RCU_SCHEDULER_INACTIVE) {
> +		might_sleep();
>  		return false;
> -	might_sleep();  /* Check for RCU read-side critical section. */
> +	}
>  	return true;
>  }
>  
> -- 
> 2.25.1
>