On Fri, Jun 30, 2017 at 09:16:07AM -0700, Paul E. McKenney wrote:
> On Fri, Jun 30, 2017 at 05:20:10PM +0200, Oleg Nesterov wrote:
> > 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...
Well, I tried documenting exactly what it did and did not do, which got
an ack from Peter.
https://marc.info/?l=linux-kernel&m=149575078313105
However, my later pull request spawned a bit of discussion:
https://marc.info/?l=linux-kernel&m=149730349001044
This discussion led me to propose strengthening spin_unlock_wait()
to act as a lock/unlock pair. This can be implemented on x86 as
an smp_mb() followed by a read-only spinloop, as shown on branch
spin_unlock_wait.2017.06.23a on my -rcu tree.
Linus was not amused, and said that if we were going to make
spin_unlock_wait() have the semantics of lock+unlock, we should just
open-code that, especially given that there are way more definitions
of spin_unlock_wait() than there are uses. He also suggested making
spin_unlock_wait() have only acquire semantics (x86 spin loop with
no memory-barrier instructions) and add explicit barriers where
required.
https://marc.info/?l=linux-kernel&m=149860012913036
I did a series for this which may be found on branch
spin_unlock_wait.2017.06.27a on my -rcu tree.
This approach was not loved by others (see later on the above thread), and
Linus's reply (which reiterated his opposition to lock+unlock semantics)
suggested the possibility of removing spin_unlock_wait() entirely.
https://marc.info/?l=linux-kernel&m=149869476911620
So I figured, in for a penny, in for a pound, and therefore did the series
that includes this patch. The most recent update (which does not yet
include your improved version) is on branch spin_unlock_wait.2017.06.30b
of my -rcu tree.
Hey, you asked! ;-)
Thanx, Paul
> > 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:
>
> This looks -much- better than my patch! May I have your Signed-off-by?
>
> Thanx, Paul
>
> > --- 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.
> >
