On 08/21, Oleg Nesterov wrote:
>
> > task_work_add(struct task_struct *task, struct callback_head *twork, bool
> > notify)
> > {
> > do {
> > twork->next = ACCESS_ONCE(task->task_works);
> > if (unlikely(twork->next == TWORK_EXITED))
> > return -ESRCH;
> > } while (cmp_xchg(&task->task_works, twork->next, twork));
> ^^^^^^^^^^^^^^^
>
> while (!cmp_xchg(...))
and _cancel can be simplified a bit.
OK, I actually tested the code below, seems to work.
Peter, if you think it can work for you and if you agree with
the implementation I will be happy to send the patch.
The only change outside of task_work.c is that exit_task_work()
should call task_work_run() unconditionally.
As for cmp_xchg below... Of course it is not strictly needed.
But otoh, personally I'd like to have this helper (probably
renamed) somewhere in include/linux. Perhaps this is just me,
but cmpxchg() always confuses me, and most users only need
success_or_not from cmpxchg.
Oleg.
#define cmp_xchg(ptr, _old, new) \
({ \
__typeof__(_old) old = (_old); \
cmpxchg(ptr, old, new) == old; \
})
#define TWORK_EXITED ((struct callback_head*)1)
int
task_work_add(struct task_struct *task, struct callback_head *work, bool notify)
{
do {
work->next = ACCESS_ONCE(task->task_works);
if (unlikely(work->next == TWORK_EXITED))
return -ESRCH;
} while (!cmp_xchg(&task->task_works, work->next, work));
/* test_and_set_bit() implies mb(), see tracehook_notify_resume(). */
if (notify)
set_notify_resume(task);
return 0;
}
struct callback_head *
task_work_cancel(struct task_struct *task, task_work_func_t func)
{
struct callback_head **pprev = &task->task_works;
struct callback_head *work = NULL;
unsigned long flags;
raw_spin_lock_irqsave(&task->pi_lock, flags);
if (likely(*pprev != TWORK_EXITED)) {
while ((work = *pprev)) {
read_barrier_depends();
if (work->func != func)
pprev = &work->next;
else if (cmp_xchg(pprev, work, work->next))
break;
}
}
raw_spin_unlock_irqrestore(&task->pi_lock, flags);
return work;
}
void task_work_run(void)
{
struct task_struct *task = current;
struct callback_head *work, *head, *next;
for (;;) {
raw_spin_lock_irq(&task->pi_lock);
do {
work = ACCESS_ONCE(task->task_works);
head = !work && (task->flags & PF_EXITING) ?
TWORK_EXITED : NULL;
} while (!cmp_xchg(&task->task_works, work, head));
raw_spin_unlock_irq(&task->pi_lock);
if (!work)
break;
#if PETERZ_WONT_ARGUE_AGAINST_FIFO_TOO_MUCH
head = NULL;
do {
next = work->next;
work->next = head;
head = work;
work = next;
} while (work);
work = head;
#endif
do {
next = work->next;
work->func(work);
work = next;
cond_resched();
} while (work);
}
}
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