linux-kernel - Re: [PATCH rcu 15/18] rcu: Remove useless "ret" update in rcu_gp_fqs

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Date:   Tue, 3 Aug 2021 10:10:35 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Joe Perches <joe@...ches.com>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, mingo@...nel.org, jiangshanlai@...il.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org,
        Liu Song <liu.song11@....com.cn>
Subject: Re: [PATCH rcu 15/18] rcu: Remove useless "ret" update in
 rcu_gp_fqs_loop()

On Tue, Aug 03, 2021 at 09:48:23AM -0700, Joe Perches wrote:
> On Wed, 2021-07-21 at 13:21 -0700, Paul E. McKenney wrote:
> > From: Liu Song <liu.song11@....com.cn>
> > 
> > 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.
> []
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> []
> > @@ -1960,8 +1960,8 @@ 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);
> 
> It doesn't seem this is a __must_check routine so why
> bother to cast to void ?

As a hint to the reader that discarding the return value is intentional
rather than an oversight.

							Thanx, Paul

> >  		rcu_gp_torture_wait();
> >  		WRITE_ONCE(rcu_state.gp_state, RCU_GP_DOING_FQS);
> >  		/* Locking provides needed memory barriers. */
> 
>