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>

diff --git a/kernel/task_work.c b/kernel/task_work.c
index 6ab4842..d513051 100644 (file)
--- 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);