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SubjectRe: [PATCH v12 3/3] tracing: Centralize preemptirq tracepoints and unify their usage
On Tue, Aug 07, 2018 at 08:53:54PM -0700, Joel Fernandes wrote:
> On Tue, Aug 7, 2018 at 8:44 PM, Joel Fernandes <joelaf@google.com> wrote:
> > Hi Steve,
> >
> > On Tue, Aug 7, 2018 at 7:28 PM, Steven Rostedt <rostedt@goodmis.org> wrote:
> [...]
> >>> @@ -171,8 +174,7 @@ extern void syscall_unregfunc(void);
> >>> } while ((++it_func_ptr)->func); \
> >>> } \
> >>> \
> >>> - if (rcuidle) \
> >>> - srcu_read_unlock_notrace(&tracepoint_srcu, idx);\
> >>> + srcu_read_unlock_notrace(ss, idx); \
> >>
> >> Hmm, why do we have the two different srcu handles?
> >
> > Because if the memory operations happening on the normal SRCU handle
> > (during srcu_read_lock) is interrupted by NMI, then the other handle
> > (devoted to NMI) could be used instead and not bother the interrupted
> > handle. Does that makes sense?
> >
> > When I talked to Paul few months ago about SRCU from NMI context, he
> > mentioned the per-cpu memory operations during srcu_read_lock can be
> > NMI interrupted, that's why we added that warning.
>
> So I looked more closely, __srcu_read_lock on 2 different handles may
> still be doing a this_cpu_inc on the same location..
> (sp->sda->srcu_lock_count). :-(
>
> Paul any ideas on how to solve this?

You lost me on this one. When you said "2 different handles", I assumed
that you meant two different values of "sp", which would have two
different addresses for &sp->sda->srcu_lock_count. What am I missing?

> It does start to seem like a show stopper :-(

I suppose that an srcu_read_lock_nmi() and srcu_read_unlock_nmi() could
be added, which would do atomic ops on sp->sda->srcu_lock_count. Not sure
whether this would be fast enough to be useful, but easy to provide:

int __srcu_read_lock_nmi(struct srcu_struct *sp) /* UNTESTED. */
{
int idx;

idx = READ_ONCE(sp->srcu_idx) & 0x1;
atomic_inc(&sp->sda->srcu_lock_count[idx]);
smp_mb__after_atomic(); /* B */ /* Avoid leaking critical section. */
return idx;
}

void __srcu_read_unlock_nmi(struct srcu_struct *sp, int idx)
{
smp_mb__before_atomic(); /* C */ /* Avoid leaking critical section. */
atomic_inc(&sp->sda->srcu_unlock_count[idx]);
}

With appropriate adjustments to also allow Tiny RCU to also work.

Note that you have to use _nmi() everywhere, not just in NMI handlers.
In fact, the NMI handlers are the one place you -don't- need to use
_nmi(), strangely enough.

Might be worth a try -- smp_mb__{before,after}_atomic() is a no-op on
some architectures, for example.

Thanx, Paul

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