task_work: use READ_ONCE/lockless_dereference, avoid pi_lock if !task_works

Change task_work_cancel() to use lockless_dereference(), this is what
the code really wants but we didn't have this helper when it was
written.

Also add the fast-path task->task_works == NULL check, in the likely
case this task has no pending works and we can avoid
spin_lock(task->pi_lock).

While at it, change other users of ACCESS_ONCE() to use READ_ONCE().

Link: http://lkml.kernel.org/r/20160610150042.GA13868@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Andrea Parri <parri.andrea@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 4 deletions

diff --git a/kernel/task_work.c b/kernel/task_work.c
index 6ab4842b00e8..d513051fcca2 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -29,7 +29,7 @@ task_work_add(struct task_struct *task, struct callback_head *work, bool notify)
         struct callback_head *head;
 
         do {
-                head = ACCESS_ONCE(task->task_works);
+                head = READ_ONCE(task->task_works);
                 if (unlikely(head == &work_exited))
                         return -ESRCH;
                 work->next = head;
@@ -57,6 +57,9 @@ task_work_cancel(struct task_struct *task, task_work_func_t func)
         struct callback_head **pprev = &task->task_works;
         struct callback_head *work;
         unsigned long flags;
+
+        if (likely(!task->task_works))
+                return NULL;
         /*
          * If cmpxchg() fails we continue without updating pprev.
          * Either we raced with task_work_add() which added the
@@ -64,8 +67,7 @@ task_work_cancel(struct task_struct *task, task_work_func_t func)
          * we raced with task_work_run(), *pprev == NULL/exited.
          */
         raw_spin_lock_irqsave(&task->pi_lock, flags);
-        while ((work = ACCESS_ONCE(*pprev))) {
-                smp_read_barrier_depends();
+        while ((work = lockless_dereference(*pprev))) {
                 if (work->func != func)
                         pprev = &work->next;
                 else if (cmpxchg(pprev, work, work->next) == work)
@@ -95,7 +97,7 @@ void task_work_run(void)
                  * work_exited unless the list is empty.
                  */
                 do {
-                        work = ACCESS_ONCE(task->task_works);
+                        work = READ_ONCE(task->task_works);
                         head = !work && (task->flags & PF_EXITING) ?
                                 &work_exited : NULL;
                 } while (cmpxchg(&task->task_works, work, head) != work);