Re: [tip: timers/core] hrtimer: Annotate lockless access to timer->state

[Paul E. McKenney]

On Thu, Nov 07, 2019 at 08:59:49AM -0800, Eric Dumazet wrote:
> On Thu, Nov 7, 2019 at 8:54 AM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> >
> > On Thu, Nov 07, 2019 at 08:39:42AM -0800, Eric Dumazet wrote:
> > > On Thu, Nov 7, 2019 at 8:35 AM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> > > >
> > > > On Thu, Nov 7, 2019 at 8:11 AM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> > > > >
> > > > > OK, so this is due to timer_pending() lockless access to ->entry.pprev
> > > > > to determine whether or not the timer is on the list.  New one on me!
> > > > >
> > > > > Given that use case, I don't have an objection to your patch to list.h.
> > > > >
> > > > > Except...
> > > > >
> > > > > Would it make sense to add a READ_ONCE() to hlist_unhashed()
> > > > > and to then make timer_pending() invoke hlist_unhashed()?  That
> > > > > would better confine the needed uses of READ_ONCE().
> > > >
> > > > Sounds good to me, I had the same idea but was too lazy to look at the
> > > > history of timer_pending()
> > > > to check if the pprev pointer check was really the same underlying idea.
> > >
> > > Note that forcing READ_ONCE() in hlist_unhashed() might force the compiler
> > > to read the pprev pointer twice in some cases.
> > >
> > > This was one of the reason for me to add skb_queue_empty_lockless()
> > > variant in include/linux/skbuff.h
> >
> > Ouch!
> >
> > > /**
> > >  * skb_queue_empty_lockless - check if a queue is empty
> > >  * @list: queue head
> > >  *
> > >  * Returns true if the queue is empty, false otherwise.
> > >  * This variant can be used in lockless contexts.
> > >  */
> > > static inline bool skb_queue_empty_lockless(const struct sk_buff_head *list)
> > > {
> > > return READ_ONCE(list->next) == (const struct sk_buff *) list;
> > > }
> > >
> > > So maybe add a hlist_unhashed_lockless() to clearly document why
> > > callers are using the lockless variant ?
> >
> > That sounds like a reasonable approach to me.  There aren't all that
> > many uses of hlist_unhashed(), so a name change should not be a problem.
> 
> Maybe I was not clear :  I did not rename skb_queue_empty()
> I chose to add another helper.
> 
> Contexts that can safely use skb_queue_empty() still continue to use
> it, since it might help
> the compiler to generate better code.
> 
> So If I add hlist_unhashed_lockless(), I would only use it from
> timer_pending() at first.
> 
> Then an audit of the code might reveal other potential users.

OK, yes, that approach does make more sense, and thank you for the
clarification.

							Thanx, Paul