linux-kernel - [PATCH v2 tip/core/rcu 03/10] task_work: Replace spin_unlock

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Message-Id: <1501541897-5225-3-git-send-email-paulmck@linux.vnet.ibm.com>
Date:   Mon, 31 Jul 2017 15:58:10 -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>
Subject: [PATCH v2 tip/core/rcu 03/10] task_work: Replace spin_unlock_wait() with lock/unlock pair

From: Oleg Nesterov <oleg@...hat.com>

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
task_work_run() with a spin_lock_irq() and a spin_unlock_irq() aruond
the cmpxchg() dequeue loop.  This should be safe from a performance
perspective because ->pi_lock is local to the task and because calls to
the other side of the race, task_work_cancel(), should be rare.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 kernel/task_work.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/kernel/task_work.c b/kernel/task_work.c
index d513051fcca2..836a72a66fba 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -96,20 +96,16 @@ void task_work_run(void)
 		 * work->func() can do task_work_add(), do not set
 		 * work_exited unless the list is empty.
 		 */
+		raw_spin_lock_irq(&task->pi_lock);
 		do {
 			work = READ_ONCE(task->task_works);
 			head = !work && (task->flags & PF_EXITING) ?
 				&work_exited : NULL;
 		} while (cmpxchg(&task->task_works, work, head) != work);
+		raw_spin_unlock_irq(&task->pi_lock);
 
 		if (!work)
 			break;
-		/*
-		 * Synchronize with task_work_cancel(). It can't remove
-		 * the first entry == work, cmpxchg(task_works) should
-		 * fail, but it can play with *work and other entries.
-		 */
-		raw_spin_unlock_wait(&task->pi_lock);
 
 		do {
 			next = work->next;
-- 
2.5.2