linux-kernel - Re: [PATCH] rcu: Remove unnecessary updates to ret in rcu_gp_fqs

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Message-ID: <20210629174408.GS4397@paulmck-ThinkPad-P17-Gen-1>
Date:   Tue, 29 Jun 2021 10:44:08 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Liu Song <fishland@...yun.com>
Cc:     liu.song11@....com.cn, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu: Remove unnecessary updates to ret in rcu_gp_fqs_loop

On Tue, Jun 29, 2021 at 09:55:51PM +0800, Liu Song wrote:
> From: Liu Song <liu.song11@....com.cn>
> 
> The ret value will be updated in the subsequent process,
> so there is no need to update it here.
> 
> Signed-off-by: Liu Song <liu.song11@....com.cn>

Good eyes, and it is safe to say that rcutorture would never have found
that one.  ;-)

As usual, I could not resist the urge to edit the commit log, and I also
added the "(void)" to indicate that the code intends to ignore the return
value, as it now uses elapsed time to make its decisions.

Please let me know if I messed anything up.

							Thanx, Paul

------------------------------------------------------------------------

commit 48434fc6700c31bd258713c24b78053fe47d393e
Author: Liu Song <liu.song11@....com.cn>
Date:   Tue Jun 29 21:55:51 2021 +0800

    rcu: Remove useless "ret" update in rcu_gp_fqs_loop()
    
    Within rcu_gp_fqs_loop(), the "ret" local variable is set to the
    return value from swait_event_idle_timeout_exclusive(), but "ret" is
    unconditionally overwritten later in the code.  This commit therefore
    removes this useless assignment.
    
    Signed-off-by: Liu Song <liu.song11@....com.cn>
    Signed-off-by: Paul E. McKenney <paulmck@...nel.org>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 35ed10bb95ef..a627a200a5ae 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1958,8 +1958,8 @@ static noinline_for_stack void rcu_gp_fqs_loop(void)
 		trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq,
 				       TPS("fqswait"));
 		WRITE_ONCE(rcu_state.gp_state, RCU_GP_WAIT_FQS);
-		ret = swait_event_idle_timeout_exclusive(
-				rcu_state.gp_wq, rcu_gp_fqs_check_wake(&gf), j);
+		(void)swait_event_idle_timeout_exclusive(rcu_state.gp_wq,
+				 rcu_gp_fqs_check_wake(&gf), j);
 		rcu_gp_torture_wait();
 		WRITE_ONCE(rcu_state.gp_state, RCU_GP_DOING_FQS);
 		/* Locking provides needed memory barriers. */