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

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Message-ID: <PH0PR11MB5880526CB255AFF91F0D76A2DAE79@PH0PR11MB5880.namprd11.prod.outlook.com>
Date:   Sat, 17 Dec 2022 02:08:27 +0000
From:   "Zhang, Qiang1" <qiang1.zhang@...el.com>
To:     "paulmck@...nel.org" <paulmck@...nel.org>
CC:     "frederic@...nel.org" <frederic@...nel.org>,
        "quic_neeraju@...cinc.com" <quic_neeraju@...cinc.com>,
        "joel@...lfernandes.org" <joel@...lfernandes.org>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        "linux-kernel@...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.
>

__might_sleep
   __might_resched(file, line, 0)
      rcu_sleep_check()

Does it refer to this rcu_sleep_check() ?

If so, when in the RCU_SCHEDULER_INACTIVE state,  the debug_lockdep_rcu_enabled() is always
return false, so the RCU_LOCKDEP_WARN() also does not produce an actual warning.

Thanks
Zqiang


>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
>