[PATCH v2 3/9] sched: Replace spin_unlock_wait() with lock/unlock pair

From: Paul E. McKenney
Date: Wed Jul 05 2017 - 19:32:11 EST

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@xxxxxxxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Cc: Andrea Parri <parri.andrea@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
[ 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 e91138fcde86..48a8760fedf4 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3460,8 +3460,9 @@ 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(): */