From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Joel Fernandes <joelaf@google.com>
Cc: Steven Rostedt <rostedt@goodmis.org>,
Joel Fernandes <joel@joelfernandes.org>,
LKML <linux-kernel@vger.kernel.org>,
"Cc: Android Kernel" <kernel-team@android.com>,
Boqun Feng <boqun.feng@gmail.com>,
Byungchul Park <byungchul.park@lge.com>,
Ingo Molnar <mingo@redhat.com>,
Masami Hiramatsu <mhiramat@kernel.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Namhyung Kim <namhyung@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Thomas Glexiner <tglx@linutronix.de>,
Tom Zanussi <tom.zanussi@linux.intel.com>
Subject: Re: [PATCH v12 3/3] tracing: Centralize preemptirq tracepoints and unify their usage
Date: Wed, 8 Aug 2018 06:00:41 -0700 [thread overview]
Message-ID: <20180808130041.GI24813@linux.vnet.ibm.com> (raw)
In-Reply-To: <CAJWu+oqPUuF4rFOQ8=G3+ZxXJxiiNRBYE8C=zZ93A8qA+muHEg@mail.gmail.com>
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
next prev parent reply other threads:[~2018-08-08 13:00 UTC|newest]
Thread overview: 57+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-07-30 22:24 [PATCH v12 0/3] tracing: Centralize preemptirq tracepoints and unify their usage Joel Fernandes
2018-07-30 22:24 ` [PATCH v12 1/3] lockdep: use this_cpu_ptr instead of get_cpu_var stats Joel Fernandes
2018-07-30 22:24 ` [PATCH v12 2/3] tracepoint: Make rcuidle tracepoint callers use SRCU Joel Fernandes
2018-07-30 23:10 ` Steven Rostedt
2018-08-10 15:35 ` Steven Rostedt
2018-08-10 16:32 ` Steven Rostedt
2018-07-30 22:24 ` [PATCH v12 3/3] tracing: Centralize preemptirq tracepoints and unify their usage Joel Fernandes
2018-08-06 19:50 ` Steven Rostedt
2018-08-07 0:43 ` Joel Fernandes
2018-08-07 1:43 ` Steven Rostedt
2018-08-07 13:33 ` Joel Fernandes
2018-08-07 13:49 ` Steven Rostedt
2018-08-07 14:10 ` Joel Fernandes
2018-08-07 14:34 ` Steven Rostedt
2018-08-07 14:48 ` Joel Fernandes
2018-08-07 15:09 ` Steven Rostedt
2018-08-07 15:24 ` Joel Fernandes
2018-08-07 23:45 ` Steven Rostedt
2018-08-07 23:54 ` Joel Fernandes
2018-08-08 0:48 ` Steven Rostedt
2018-08-08 1:17 ` Joel Fernandes
2018-08-08 1:55 ` Steven Rostedt
2018-08-08 2:13 ` Joel Fernandes
2018-08-08 2:28 ` Steven Rostedt
2018-08-08 3:44 ` Joel Fernandes
2018-08-08 3:53 ` Joel Fernandes
2018-08-08 5:06 ` Joel Fernandes
2018-08-08 12:46 ` Steven Rostedt
2018-08-08 13:03 ` Paul E. McKenney
2018-08-08 13:07 ` Steven Rostedt
2018-08-08 14:33 ` Paul E. McKenney
2018-08-08 14:49 ` Steven Rostedt
2018-08-08 15:05 ` Paul E. McKenney
2018-08-08 15:23 ` Steven Rostedt
2018-08-08 16:02 ` Paul E. McKenney
2018-08-08 16:24 ` Steven Rostedt
2018-08-08 17:21 ` Paul E. McKenney
2018-08-08 13:00 ` Paul E. McKenney [this message]
2018-08-08 14:10 ` Joel Fernandes
2018-08-08 14:49 ` Paul E. McKenney
2018-08-08 19:24 ` Joel Fernandes
2018-08-08 20:18 ` Paul E. McKenney
2018-08-08 22:15 ` Joel Fernandes
2018-08-08 22:47 ` Paul E. McKenney
2018-08-09 12:18 ` joel
2018-08-08 14:27 ` Steven Rostedt
2018-08-08 14:42 ` Paul E. McKenney
2018-08-08 15:27 ` Steven Rostedt
2018-08-08 16:03 ` Paul E. McKenney
2018-08-02 14:55 ` [PATCH v12 0/3] " Masami Hiramatsu
2018-08-03 2:57 ` Joel Fernandes
2018-08-03 7:23 ` Masami Hiramatsu
2018-08-04 4:51 ` Joel Fernandes
2018-08-05 16:46 ` Joel Fernandes
2018-08-06 2:07 ` Masami Hiramatsu
2018-08-06 15:24 ` Joel Fernandes
2018-08-03 7:34 ` Masami Hiramatsu
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