On Fri, Jun 30, 2017 at 03:50:33PM -0400, Alan Stern wrote:
> On Fri, 30 Jun 2017, Oleg Nesterov wrote:
>
> > On 06/30, Paul E. McKenney wrote:
> > >
> > > On Fri, Jun 30, 2017 at 05:20:10PM +0200, Oleg Nesterov wrote:
> > > >
> > > > 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,
> > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> >
> > if it was not clear, I tried to say "why do we want to _remove_
> > unlock_wait".
> >
> > > > 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:
> > >
> > > This looks -much- better than my patch! May I have your Signed-off-by?
> >
> > Only if you promise to replace all RCU flavors with a single simple
> > implementation
> > based on rwlock ;)
> >
> > Seriously, of course I won't argue, and it seems that nobody except me likes
> > this primitive, but to me spin_unlock_wait() looks like synchronize_rcu(()
> > and
> > sometimes it makes sense.
>
> If it looks like synchronize_rcu(), why not actually use
> synchronize_rcu()?
My guess is that the latencies of synchronize_rcu() don't suit his needs.
When the lock is not held, spin_unlock_wait() is quite fast, even
compared to expedited grace periods.
Thanx, Paul
> Alan Stern
>
> > Including this particular case. task_work_run() is going to flush/destroy
> > the
> > ->task_works list, so it needs to wait until all currently executing
> > "readers"
> > (task_work_cancel()'s which have started before ->task_works was updated)
> > have
> > completed.
>
