Subject: Re: [PATCH RFC 02/26] task_work: Replace spin_unlock_wait() with lock/unlock pair

From: "Paul E. McKenney" <[email-masked]>
Date: Fri, 30 Jun 2017 10:21:01 -0700

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.
> >