linux-kernel - [PATCH tip/core/rcu 4/8] rcu: Remove preemption disabling from expedited CPU selection

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Message-Id: <20190109214142.4428-4-paulmck@linux.ibm.com>
Date:   Wed,  9 Jan 2019 13:41:38 -0800
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     linux-kernel@...r.kernel.org
Cc:     mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.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,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>
Subject: [PATCH tip/core/rcu 4/8] rcu: Remove preemption disabling from expedited CPU selection

It turns out that it is queue_delayed_work_on() rather than
queue_work_on() that has difficulties when used concurrently with
CPU-hotplug removal operations.  It is therefore unnecessary to protect
CPU identification and queue_work_on() with preempt_disable().

This commit therefore removes the preempt_disable() and preempt_enable()
from sync_rcu_exp_select_cpus(), which has the further benefit of reducing
the number of changes that must be maintained in the -rt patchset.

Reported-by: Thomas Gleixner <tglx@...utronix.de>
Reported-by: Sebastian Siewior <bigeasy@...utronix.de>
Suggested-by: Boqun Feng <boqun.feng@...il.com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>
---
 kernel/rcu/tree_exp.h | 2 --
 1 file changed, 2 deletions(-)

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 928fe5893a57..f42c999f4d2d 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -449,7 +449,6 @@ static void sync_rcu_exp_select_cpus(smp_call_func_t func)
 			continue;
 		}
 		INIT_WORK(&rnp->rew.rew_work, sync_rcu_exp_select_node_cpus);
-		preempt_disable();
 		cpu = find_next_bit(&rnp->ffmask, BITS_PER_LONG, -1);
 		/* If all offline, queue the work on an unbound CPU. */
 		if (unlikely(cpu > rnp->grphi - rnp->grplo))
@@ -457,7 +456,6 @@ static void sync_rcu_exp_select_cpus(smp_call_func_t func)
 		else
 			cpu += rnp->grplo;
 		queue_work_on(cpu, rcu_par_gp_wq, &rnp->rew.rew_work);
-		preempt_enable();
 		rnp->exp_need_flush = true;
 	}
 
-- 
2.17.1