Re: [PATCH] doc: Update wake_up() & co. memory-barrier guarantees

Andrea Parri
2018-06-25T14:56:25Z

On Mon, Jun 25, 2018 at 04:18:30PM +0200, Peter Zijlstra wrote:
> On Mon, Jun 25, 2018 at 03:16:43PM +0200, Andrea Parri wrote:
> > > > A concrete example being the store-buffering pattern reported in [1].
> > >
> > > Well, that example only needs a store->load barrier. It so happens
> > > smp_mb() is the only one actually doing that, but imagine we had a
> > > weaker barrier that did just that, one that did not imply the full
> > > transitivity smp_mb() does.
> > >
> > > Then the example from [1] could use that weaker thing.
> >
> > Absolutely (and that would be "fence w,r" on RISC-V, IIUC).
>
> Ah cute. What is the transitivity model of those "fence" instructions? I
> see their smp_mb() is "fence rw,rw" and smp_mb() must be RSsc. Otoh
> their smp_wmb() is "fence w,w" which is only only required to be RCpc.
>
> So what does RISC-V do for "w,w" and "w,r" like things?

I'd defer to Daniel (in Cc:) for this ;-) I simply checked the SB pattern
plus w,r fences against the following models:

http://www.cl.cam.ac.uk/~sf502/regressions/rmem/
http://moscova.inria.fr/~maranget/cats7/riscv/

>
> > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > index a98d54cd5535..8374d01b2820 100644
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -1879,7 +1879,9 @@ static void ttwu_queue(struct task_struct *p, int cpu, int wake_flags)
> > > * C) LOCK of the rq(c1)->lock scheduling in task
> > > *
> > > * Transitivity guarantees that B happens after A and C after B.
> > > - * Note: we only require RCpc transitivity.
> > > + * Note: we only require RCpc transitivity for these cases,
> > > + * but see smp_mb__after_spinlock() for why rq->lock is required
> > > + * to be RCsc.
> > > * Note: the CPU doing B need not be c0 or c1
> >
> > FWIW, we discussed this pattern here:
> >
> > http://lkml.kernel.org/r/20171018010748.GA4017@andrea
>
> That's not the patter from smp_mb__after_spinlock(), right? But the
> other two from this comment.

Indeed.

>
> > > @@ -1966,6 +1969,10 @@ static void ttwu_queue(struct task_struct *p, int cpu, int wake_flags)
> > > * Atomic against schedule() which would dequeue a task, also see
> > > * set_current_state().
> > > *
> > > + * Implies at least a RELEASE such that the waking task is guaranteed to
> > > + * observe the stores to the wait-condition; see set_task_state() and the
> > > + * Program-Order constraints.
> >
> > [s/set_task_task/set_current_state ?]
>
> Yes, we got rid of set_task_state(), someone forgot to tell my fingers
> :-)
>
> > I'd stick to "Implies/Executes at least a full barrier"; this is in fact
> > already documented in the function body:
> >
> > /*
> > * If we are going to wake up a thread waiting for CONDITION we
> > * need to ensure that CONDITION=1 done by the caller can not be
> > * reordered with p->state check below. This pairs with mb() in
> > * set_current_state() the waiting thread does.
> > */
> >
> > (this is, again, that "store->load barrier"/SB).
> >
> > I'll try to integrate these changes in v2, if there is no objection.
>
> Thanks!

Ah, before sending v2, I'd really appreciate some comments on the XXXs
I've added to wait_woken() as I'm not sure I understand the pattern in
questions. For example, the second comment says:

/*
* The below implies an smp_mb(), it too pairs with the smp_wmb() from
* woken_wake_function() such that we must either observe the wait
* condition being true _OR_ WQ_FLAG_WOKEN such that we will not miss
* an event.
*/

From this I understand:

wq_entry->flags &= ~WQ_FLAG_WOKEN; condition = true;
smp_mb() // B smp_wmb(); // C
[next iteration of the loop] wq_entry->flags |= WQ_FLAG_WOKEN;
if (condition)
break;

BUG_ON(!condition && !(wq_entry->flags & WQ_FLAG_WOKEN))

IOW, this is an R-like pattern: if this is the case, the smp_wmb() does
_not_ prevent the BUG_ON() from firing; according to LKMM (and powerpc)
a full barrier would be needed.

Same RFC for the first comment:

/*
* The above implies an smp_mb(), which matches with the smp_wmb() from
* woken_wake_function() such that if we observe WQ_FLAG_WOKEN we must
* also observe all state before the wakeup.
*/

What is the corresponding snippet & BUG_ON()?

Andrea