Messages in this thread Patch in this message | | | Date | Mon, 8 Oct 2012 14:38:15 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH] task_work: avoid unneeded cmpxchg() in task_work_run() |
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On 10/08, Lai Jiangshan wrote: > > We only require cmpxchg()&retry when task is exiting. > xchg() is enough in other cases like original code in ac3d0da8.
Yes, we can probably do xchg/cmpxchg depending on NULL/work_exited.
Not sure it makes sense to complicate the code though. Is xchg() really faster than cmpxchg?
> Also remove the inner loop
Yes, it is not really needed, only for readability. "do while (!cmpxchg)" can be replaced with "if (!cmpxchg) continue".
> --- a/kernel/task_work.c > +++ b/kernel/task_work.c > @@ -56,14 +56,13 @@ void task_work_run(void) > * work->func() can do task_work_add(), do not set > * work_exited unless the list is empty. > */ > - do { > - work = ACCESS_ONCE(task->task_works); > - head = !work && (task->flags & PF_EXITING) ? > - &work_exited : NULL; > - } while (cmpxchg(&task->task_works, work, head) != work); > - > - if (!work) > + if (!ACCESS_ONCE(task->task_works) ||
ACCESS_ONCE() looks confusing. It is not needed with this patch.
> + !(work = xchg(&task->task_works, NULL))) { > + if ((task->flags & PF_EXITING) && > + cmpxchg(&task->task_works, NULL, &work_exited)) > + continue; > break; > + }
I think the patch is correct.
But the code looks more complex, and the only advantage is that non-exiting task does xchg() instead of cmpxchg(). Not sure this worth the trouble, in this case task_work_run() will likey run the callbacks (the caller checks ->task_works != NULL), I do not think this can add any noticeable speedup.
But, as for correctness, Reviewed-by: Oleg Nesterov <oleg@redhat.com>
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