linux-kernel - Re: [PATCH 11/16] rcu: Check for spurious wakeup using return value

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Message-ID: <20140723122608.GL11241@linux.vnet.ibm.com>
Date:	Wed, 23 Jul 2014 05:26:08 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Pranith Kumar <bobby.prani@...il.com>
Cc:	Josh Triplett <josh@...htriplett.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	"open list:READ-COPY UPDATE..." <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 11/16] rcu: Check for spurious wakeup using return value

On Wed, Jul 23, 2014 at 01:09:48AM -0400, Pranith Kumar wrote:
> When the gp_kthread wakes up from the wait event, it returns 0 if the wake up is
> due to the condition having been met. This commit checks this return value
> for a spurious wake up before calling rcu_gp_init().
> 
> Signed-off-by: Pranith Kumar <bobby.prani@...il.com>

How does this added check help?  I don't see that it does.  If the flag
is set, we want to wake up.  If we get a spurious wakeup, but then the
flag gets set before we actually wake up, we still want to wake up.

So sorry, but no.

							Thanx, Paul

PS.  BTW, the ones that I am not immediately responding to are those
     that I will recheck later on and possibly accept.

> ---
>  kernel/rcu/tree.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index f8847d9..3b20ad2 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1790,11 +1790,11 @@ static int __noreturn rcu_gp_kthread(void *arg)
>  					       ACCESS_ONCE(rsp->gpnum),
>  					       TPS("reqwait"));
>  			rsp->gp_state = RCU_GP_WAIT_GPS;
> -			wait_event_interruptible(rsp->gp_wq,
> +			ret = wait_event_interruptible(rsp->gp_wq,
>  						 ACCESS_ONCE(rsp->gp_flags) &
>  						 RCU_GP_FLAG_INIT);
>  			/* Locking provides needed memory barrier. */
> -			if (rcu_gp_init(rsp))
> +			if (!ret && rcu_gp_init(rsp))
>  				break;
>  			cond_resched();
>  			flush_signals(current);
> -- 
> 2.0.0.rc2
> 

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