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[PATCH tip/core/rcu 4/9] completion: Replace spin_unlock_wait() with lock/unlock pair

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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
completion_done() with spin_lock() followed immediately by spin_unlock().
This should be safe from a performance perspective because the lock
will be held only the wakeup happens really quickly.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Andrea Parri <parri.andrea@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
---
 kernel/sched/completion.c | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/completion.c b/kernel/sched/completion.c
index 13fc5ae9bf2f..3feb218171a9 100644
--- a/kernel/sched/completion.c
+++ b/kernel/sched/completion.c
@@ -307,14 +307,9 @@ bool completion_done(struct completion *x)
 	 * If ->done, we need to wait for complete() to release ->wait.lock
 	 * otherwise we can end up freeing the completion before complete()
 	 * is done referencing it.
-	 *
-	 * The RMB pairs with complete()'s RELEASE of ->wait.lock and orders
-	 * the loads of ->done and ->wait.lock such that we cannot observe
-	 * the lock before complete() acquires it while observing the ->done
-	 * after it's acquired the lock.
 	 */
-	smp_rmb();
-	spin_unlock_wait(&x->wait.lock);
+	spin_lock_irq(&x->wait.lock);
+	spin_unlock_irq(&x->wait.lock);
 	return true;
 }
 EXPORT_SYMBOL(completion_done);
-- 
2.5.2

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[PATCH tip/core/rcu 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-25 00:20 +0200
  [PATCH tip/core/rcu 1/9] net/netfilter/nf_conntrack_core: Fix net_conntrack_lock() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-25 00:20 +0200
  [PATCH tip/core/rcu 3/9] sched: Replace spin_unlock_wait() with lock/unlock pair "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-25 00:20 +0200
  [PATCH tip/core/rcu 5/9] exit: Replace spin_unlock_wait() with lock/unlock pair "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-25 00:20 +0200
  [PATCH tip/core/rcu 2/9] task_work: Replace spin_unlock_wait() with lock/unlock pair "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-25 00:20 +0200
  [PATCH tip/core/rcu 8/9] locking: Remove spin_unlock_wait() generic definitions "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-25 00:20 +0200
  [PATCH tip/core/rcu 7/9] drivers/ata: Replace spin_unlock_wait() with lock/unlock pair "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-25 00:20 +0200
  [PATCH tip/core/rcu 9/9] arch: Remove spin_unlock_wait() arch-specific definitions "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-25 00:20 +0200
  [PATCH tip/core/rcu 6/9] ipc: Replace spin_unlock_wait() with lock/unlock pair "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-25 00:20 +0200
  [PATCH tip/core/rcu 4/9] completion: Replace spin_unlock_wait() with lock/unlock pair "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-25 00:20 +0200

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