lkml.org 
[lkml]   [2022]   [Aug]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[PATCH] task_work: Use try_cmpxchg in task_work_add, task_work_cancel_match and task_work_run
Date
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.

Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
---
kernel/task_work.c | 16 +++++++++-------
1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/kernel/task_work.c b/kernel/task_work.c
index dff75bcde151..065e1ef8fc8d 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -47,12 +47,12 @@ int task_work_add(struct task_struct *task, struct callback_head *work,
/* 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_struct *task,
* 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;
--
2.37.1
\
 
 \ /
  Last update: 2022-08-23 19:44    [W:0.025 / U:1.496 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site