linux-kernel - [PATCH tip/core/rcu 3/9] sched: Replace spin_unlock

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Date:   Mon, 24 Jul 2017 15:13:03 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.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,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH tip/core/rcu 3/9] sched: Replace spin_unlock_wait() with lock/unlock pair

There is no agreed-upon definition of spin_unlock_wait()'s semantics,
and it appears that all callers could do just as well with a lock/unlock
pair.  This commit therefore replaces the spin_unlock_wait() call in
do_task_dead() with spin_lock() followed immediately by spin_unlock().
This should be safe from a performance perspective because the lock is
this tasks ->pi_lock, and this is called only after the task exits.

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Will Deacon <will.deacon@....com>
Cc: Alan Stern <stern@...land.harvard.edu>
Cc: Andrea Parri <parri.andrea@...il.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
[ paulmck: Replace leading smp_mb() with smp_mb__before_spinlock(),
  courtesy of Arnd Bergmann's noting its odd location. ]
---
 kernel/sched/core.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 17c667b427b4..1179111d82a1 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3352,8 +3352,9 @@ void __noreturn do_task_dead(void)
 	 * To avoid it, we have to wait for releasing tsk->pi_lock which
 	 * is held by try_to_wake_up()
 	 */
-	smp_mb();
-	raw_spin_unlock_wait(&current->pi_lock);
+	smp_mb__before_spinlock();
+	raw_spin_lock_irq(&current->pi_lock);
+	raw_spin_unlock_irq(&current->pi_lock);
 
 	/* Causes final put_task_struct in finish_task_switch(): */
 	__set_current_state(TASK_DEAD);
-- 
2.5.2