The patch titled
Subject: task_work: use try_cmpxchg in task_work_add, task_work_cancel_match and task_work_run
has been added to the -mm mm-nonmm-unstable branch. Its filename is
task_work-use-try_cmpxchg-in-task_work_add-task_work_cancel_match-and-task_work_run.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/task_work-use-try_cmpxchg-in-task_work_add-task_work_cancel_match-and-task_work_run.patch
This patch will later appear in the mm-nonmm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Uros Bizjak <ubizjak@xxxxxxxxx>
Subject: task_work: use try_cmpxchg in task_work_add, task_work_cancel_match and task_work_run
Date: Tue, 23 Aug 2022 17:26:32 +0200
Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in
task_work_add, task_work_cancel_match and task_work_run. x86 CMPXCHG
instruction returns success in ZF flag, so this change saves a compare
after cmpxchg (and related move instruction in front of cmpxchg).
Also, atomic_try_cmpxchg implicitly assigns old *ptr value to "old"
when cmpxchg fails, enabling further code simplifications.
The patch avoids extra memory read in case cmpxchg fails.
Link: https://lkml.kernel.org/r/20220823152632.4517-1-ubizjak@xxxxxxxxx
Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
kernel/task_work.c | 16 +++++++++-------
1 file changed, 9 insertions(+), 7 deletions(-)
--- a/kernel/task_work.c~task_work-use-try_cmpxchg-in-task_work_add-task_work_cancel_match-and-task_work_run
+++ a/kernel/task_work.c
@@ -47,12 +47,12 @@ int task_work_add(struct task_struct *ta
/* record the work call stack in order to print it in KASAN reports */
kasan_record_aux_stack(work);
+ head = READ_ONCE(task->task_works);
do {
- head = READ_ONCE(task->task_works);
if (unlikely(head == &work_exited))
return -ESRCH;
work->next = head;
- } while (cmpxchg(&task->task_works, head, work) != head);
+ } while (!try_cmpxchg(&task->task_works, &head, work));
switch (notify) {
case TWA_NONE:
@@ -100,10 +100,12 @@ task_work_cancel_match(struct task_struc
* we raced with task_work_run(), *pprev == NULL/exited.
*/
raw_spin_lock_irqsave(&task->pi_lock, flags);
- while ((work = READ_ONCE(*pprev))) {
- if (!match(work, data))
+ work = READ_ONCE(*pprev);
+ while (work) {
+ if (!match(work, data)) {
pprev = &work->next;
- else if (cmpxchg(pprev, work, work->next) == work)
+ work = READ_ONCE(*pprev);
+ } else if (try_cmpxchg(pprev, &work, work->next))
break;
}
raw_spin_unlock_irqrestore(&task->pi_lock, flags);
@@ -151,16 +153,16 @@ void task_work_run(void)
* work->func() can do task_work_add(), do not set
* work_exited unless the list is empty.
*/
+ work = READ_ONCE(task->task_works);
do {
head = NULL;
- work = READ_ONCE(task->task_works);
if (!work) {
if (task->flags & PF_EXITING)
head = &work_exited;
else
break;
}
- } while (cmpxchg(&task->task_works, work, head) != work);
+ } while (!try_cmpxchg(&task->task_works, &work, head));
if (!work)
break;
_
Patches currently in -mm which might be from ubizjak@xxxxxxxxx are
llist-use-try_cmpxchg-in-llist_add_batch-and-llist_del_first.patch
epoll-use-try_cmpxchg-in-list_add_tail_lockless.patch
buffer-use-try_cmpxchg-in-discard_buffer.patch
aio-use-atomic_try_cmpxchg-in-__get_reqs_available.patch
iversion-use-atomic64_try_cmpxchg.patch
bitops-use-try_cmpxchg-in-set_mask_bits-and-bit_clear_unless.patch
task_work-use-try_cmpxchg-in-task_work_add-task_work_cancel_match-and-task_work_run.patch
