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SubjectRe: [PATCH RFC 02/26] task_work: Replace spin_unlock_wait() with lock/unlock pair
On 06/30, Paul E. McKenney wrote:
>
> > > + raw_spin_lock_irq(&task->pi_lock);
> > > + raw_spin_unlock_irq(&task->pi_lock);
>
> I agree that the spin_unlock_wait() implementations would avoid the
> deadlock with an acquisition from an interrupt handler, while also
> avoiding the need to momentarily disable interrupts. The ->pi_lock is
> a per-task lock, so I am assuming (perhaps naively) that contention is
> not a problem. So is the overhead of interrupt disabling likely to be
> noticeable here?

I do not think the overhead will be noticeable in this particular case.

But I am not sure I understand why do we want to unlock_wait. Yes I agree,
it has some problems, but still...

The code above looks strange for me. If we are going to repeat this pattern
the perhaps we should add a helper for lock+unlock and name it unlock_wait2 ;)

If not, we should probably change this code more:

--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -96,20 +96,16 @@ void task_work_run(void)
* work->func() can do task_work_add(), do not set
* work_exited unless the list is empty.
*/
+ raw_spin_lock_irq(&task->pi_lock);
do {
work = READ_ONCE(task->task_works);
head = !work && (task->flags & PF_EXITING) ?
&work_exited : NULL;
} while (cmpxchg(&task->task_works, work, head) != work);
+ raw_spin_unlock_irq(&task->pi_lock);

if (!work)
break;
- /*
- * Synchronize with task_work_cancel(). It can't remove
- * the first entry == work, cmpxchg(task_works) should
- * fail, but it can play with *work and other entries.
- */
- raw_spin_unlock_wait(&task->pi_lock);

do {
next = work->next;
performance-wise this is almost the same, and if we do not really care about
overhead we can simplify the code: this way it is obvious that we can't race
with task_work_cancel().

Oleg.

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