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Date:   Tue, 3 Aug 2021 10:10:35 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Joe Perches <joe@perches.com>
Cc:     rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com,
        akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
        josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org,
        rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com,
        fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org,
        Liu Song <liu.song11@zte.com.cn>
Subject: Re: [PATCH rcu 15/18] rcu: Remove useless "ret" update in
 rcu_gp_fqs_loop()
Message-ID: <20210803171035.GU4397@paulmck-ThinkPad-P17-Gen-1>
Reply-To: paulmck@kernel.org
References: <20210721202042.GA1472052@paulmck-ThinkPad-P17-Gen-1>
 <20210721202127.2129660-15-paulmck@kernel.org>
 <5a494dd8d0e04030204f61067cb1010ef0c78f27.camel@perches.com>
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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@zte.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. */
> 
>