Re: [PATCH tip/sched/core] Add comments to aid in safer usage of swake_up.

Paul E. McKenney
2017-06-14T03:58:52Z

On Tue, Jun 13, 2017 at 09:15:47PM -0400, Steven Rostedt wrote:
> On Tue, 13 Jun 2017 16:42:05 -0700
> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
>
> > On Tue, Jun 13, 2017 at 07:23:08PM -0400, Steven Rostedt wrote:
> > > On Fri, 9 Jun 2017 05:45:54 -0700
> > > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> > >
> > > > On Fri, Jun 09, 2017 at 09:19:57AM +0200, Peter Zijlstra wrote:
> > > > > On Thu, Jun 08, 2017 at 08:25:46PM -0700, Krister Johansen wrote:
> > > > > > The behavior of swake_up() differs from that of wake_up(), and from the
> > > > > > swake_up() that came from RT linux. A memory barrier, or some other
> > > > > > synchronization, is needed prior to a swake_up so that the waiter sees
> > > > > > the condition set by the waker, and so that the waker does not see an
> > > > > > empty wait list.
> > > > >
> > > > > Urgh.. let me stare at that. But it sounds like the wrong solution since
> > > > > we wanted to keep the wait and swait APIs as close as possible.
> > > >
> > > > But don't they both need some sort of ordering, be it memory barriers or
> > > > locking, to handle the case where the wait/swait doesn't actually sleep?
> > > >
> > >
> > > Looking at an RCU example, and assuming that ordering can move around
> > > within a spin lock, and that changes can leak into a spin lock region
> > > from both before and after. Could we have:
> > >
> > > (looking at __call_rcu_core() and rcu_gp_kthread()
> > >
> > > CPU0 CPU1
> > > ---- ----
> > > __call_rcu_core() {
> > >
> > > spin_lock(rnp_root)
> > > need_wake = __rcu_start_gp() {
> > > rcu_start_gp_advanced() {
> > > gp_flags = FLAG_INIT
> > > }
> > > }
> > >
> > > rcu_gp_kthread() {
> > > swait_event_interruptible(wq,
> > > gp_flags & FLAG_INIT) {
> > > spin_lock(q->lock)
> > >
> > > *fetch wq->task_list here! *
> > >
> > > list_add(wq->task_list, q->task_list)
> > > spin_unlock(q->lock);
> > >
> > > *fetch old value of gp_flags here *
> >
> > Both reads of ->gp_flags are READ_ONCE(), so having seen the new value
> > in swait_event_interruptible(), this task/CPU cannot see the old value
> > from some later access. You have to have accesses to two different
> > variables to require a memory barrier (at least assuming consistent use
> > of READ_ONCE(), WRITE_ONCE(), or equivalent).
>
> If I'm not mistaken, READ_ONCE() and WRITE_ONCE() is just volatiles
> added. The compiler may not leak or move the the fetches, but what
> about the hardware?

The hardware cannot move the references if both references are in the
same thread and to the same variable, which is the case with ->gp_flags.

> A spin_lock() only needs to make sure what is after it does not leak
> before it.
>
> A spin_unlock() only needs to make sure what is before it must not leak
> after it.

Both true, with the exception of a spin_is_locked() to that same
lock variable, which cannot be reordered with either spin_lock() or
spin_unlock() in either direction.

> From my understandings of reading memory-barrier.txt, there's no
> guarantees that the hardware doesn't let reads or writes that happen
> before a spin_lock() happen after it. Nor does it guarantee that reads
> or writes that happen after a spin_unlock() doesn't happen before it.
>
> The spin_locks only need to protect the inside of the critical section,
> not the outside of it leaking in.

Again, quite true.

> I'm looking at this in particular:
>
> ====
> (1) ACQUIRE operation implication:
>
> Memory operations issued after the ACQUIRE will be completed after the
> ACQUIRE operation has completed.
>
> Memory operations issued before the ACQUIRE may be completed after
> the ACQUIRE operation has completed. An smp_mb__before_spinlock(),
> combined with a following ACQUIRE, orders prior stores against
> subsequent loads and stores. Note that this is weaker than smp_mb()!
> The smp_mb__before_spinlock() primitive is free on many architectures.
>
> (2) RELEASE operation implication:
>
> Memory operations issued before the RELEASE will be completed before the
> RELEASE operation has completed.
>
> Memory operations issued after the RELEASE may be completed before the
> RELEASE operation has completed.
> ====

And here is the part you also need to look at:

====

(*) Overlapping loads and stores within a particular CPU will appear to be
ordered within that CPU. This means that for:

a = READ_ONCE(*X); WRITE_ONCE(*X, b);

the CPU will only issue the following sequence of memory operations:

a = LOAD *X, STORE *X = b

And for:

WRITE_ONCE(*X, c); d = READ_ONCE(*X);

the CPU will only issue:

STORE *X = c, d = LOAD *X

(Loads and stores overlap if they are targeted at overlapping pieces of
memory).

====

This section needs some help -- the actual guarantee is stronger, that
all CPUs will agree on the order of volatile same-sized aligned accesses
to a given single location. So if a previous READ_ONCE() sees the new
value, any subsequent READ_ONCE() from that same variable is guaranteed
to also see the new value (or some later value).

Does that help, or am I missing something here?

Thanx, Paul

> -- Steve
>
>
> >
> > > spin_unlock(rnp_root)
> > >
> > > rcu_gp_kthread_wake() {
> > > swake_up(wq) {
> > > swait_active(wq) {
> > > list_empty(wq->task_list)
> > >
> > > } * return false *
> > >
> > > if (condition) * false *
> > > schedule();
> > >
> > > Looks like a memory barrier is missing. Perhaps we should slap on into
> > > swait_active()? I don't think it is wise to let users add there own, as
> > > I think we currently have bugs now.
> >
> > I -know- I have bugs now. ;-)
> >
> > But I don't believe this is one of them. Or am I getting confused?
> >
> > Thanx, Paul
>