[PATCH] task_work_run: don't take ->pi_lock unconditionally

Oleg Nesterov
2020-02-18T15:50:27Z

As Peter pointed out, task_work() can avoid ->pi_lock and cmpxchg()
if task->task_works == NULL && !PF_EXITING.

And in fact the only reason why task_work_run() needs ->pi_lock is
the possible race with task_work_cancel(), we can optimize this code
and make the locking more clear.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
kernel/task_work.c | 18 ++++++++++++++----
1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/kernel/task_work.c b/kernel/task_work.c
index 0fef395..825f282 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -97,16 +97,26 @@ 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 {
+ head = NULL;
work = READ_ONCE(task->task_works);
- head = !work && (task->flags & PF_EXITING) ?
- &work_exited : NULL;
+ if (!work) {
+ if (task->flags & PF_EXITING)
+ head = &work_exited;
+ else
+ break;
+ }
} 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 not remove
+ * the first entry == work, cmpxchg(task_works) must fail.
+ * But it can remove another entry from the ->next list.
+ */
+ raw_spin_lock_irq(&task->pi_lock);
+ raw_spin_unlock_irq(&task->pi_lock);

do {
next = work->next;
--
2.5.0