task_work: 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
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@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files 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;