On 08/17, Oleg Nesterov wrote:
>
> On 08/17, Oleg Nesterov wrote:
> >
> > On 08/16, Peter Zijlstra wrote:
> > >
> > > write_lock_irq(&tasklist_lock)
> > > task_lock(parent) parent->alloc_lock
> >
> > And this is already wrong. See the comment above task_lock().
> >
> > > And since it_lock is IRQ-safe and alloc_lock isn't, you've got the IRQ
> > > inversion deadlock reported.
> >
> > Yes. Or, IOW, write_lock(tasklist) is IRQ-safe and thus it can't nest
> > with alloc_lock.
> >
> > > David, Al, anybody want to have a go at fixing this?
> >
> > I still think that task_work_add() should synhronize with exit_task_work()
> > itself and fail if necessary. But I wasn't able to convince Al ;)
>
> And this is my old patch: http://marc.info/?l=linux-kernel&m=134082268721700
> It should be re-diffed of course.
Something like below. Uncompiled/untested, I need to re-check and test.
Now we can remove that task_lock() and rely on task_work_add().
Al, what do you think?
Oleg.
--- x/include/linux/task_work.h
+++ x/include/linux/task_work.h
@@ -18,8 +18,7 @@ void task_work_run(void);
static inline void exit_task_work(struct task_struct *task)
{
- if (unlikely(task->task_works))
- task_work_run();
+ task_work_run();
}
#endif /* _LINUX_TASK_WORK_H */
--- x/kernel/task_work.c
+++ x/kernel/task_work.c
@@ -2,29 +2,35 @@
#include <linux/task_work.h>
#include <linux/tracehook.h>
+#define TWORK_EXITED ((struct callback_head *)1)
+
int
task_work_add(struct task_struct *task, struct callback_head *twork, bool
notify)
{
struct callback_head *last, *first;
unsigned long flags;
+ int err = -ESRCH;
/*
- * Not inserting the new work if the task has already passed
- * exit_task_work() is the responisbility of callers.
+ * We must not insert the new work if the exiting task has already
+ * passed task_work_run().
*/
raw_spin_lock_irqsave(&task->pi_lock, flags);
- last = task->task_works;
- first = last ? last->next : twork;
- twork->next = first;
- if (last)
- last->next = twork;
- task->task_works = twork;
+ if (likely(task->task_works != TWORK_EXITED) {
+ last = task->task_works;
+ first = last ? last->next : twork;
+ twork->next = first;
+ if (last)
+ last->next = twork;
+ task->task_works = twork;
+ err = 0;
+ }
raw_spin_unlock_irqrestore(&task->pi_lock, flags);
/* test_and_set_bit() implies mb(), see tracehook_notify_resume(). */
- if (notify)
+ if (!err && notify)
set_notify_resume(task);
- return 0;
+ return err;
}
struct callback_head *
@@ -35,7 +41,7 @@ task_work_cancel(struct task_struct *tas
raw_spin_lock_irqsave(&task->pi_lock, flags);
last = task->task_works;
- if (last) {
+ if (last && last != TWORK_EXITED) {
struct callback_head *q = last, *p = q->next;
while (1) {
if (p->func == func) {
@@ -63,7 +69,12 @@ void task_work_run(void)
while (1) {
raw_spin_lock_irq(&task->pi_lock);
p = task->task_works;
- task->task_works = NULL;
+ /*
+ * twork->func() can do task_work_add(), do not
+ * set TWORK_EXITED until the list becomes empty.
+ */
+ task->task_works = (!p && (task->flags & PF_EXITING))
+ ? TWORK_EXITED : NULL;
raw_spin_unlock_irq(&task->pi_lock);
if (unlikely(!p))
--
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