The patch titled
Subject: task_work: use READ_ONCE/lockless_dereference, avoid pi_lock if !task_works
has been removed from the -mm tree. Its filename was
task_work-use-read_once-lockless_dereference-avoid-pi_lock-if-task_works.patch
This patch was dropped because it was merged into mainline or a subsystem tree
------------------------------------------------------
From: Oleg Nesterov <oleg@xxxxxxxxxx>
Subject: 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@xxxxxxxxxx
Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Andrea Parri <parri.andrea@xxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
kernel/task_work.c | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)
diff -puN kernel/task_work.c~task_work-use-read_once-lockless_dereference-avoid-pi_lock-if-task_works kernel/task_work.c
--- a/kernel/task_work.c~task_work-use-read_once-lockless_dereference-avoid-pi_lock-if-task_works
+++ a/kernel/task_work.c
@@ -29,7 +29,7 @@ task_work_add(struct task_struct *task,
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 *tas
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 *tas
* 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);
_
Patches currently in -mm which might be from oleg@xxxxxxxxxx are
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