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Date: Wed, 8 Aug 2018 09:07:24 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: Joel Fernandes <joelaf@...gle.com>,
Joel Fernandes <joel@...lfernandes.org>,
LKML <linux-kernel@...r.kernel.org>,
"Cc: Android Kernel" <kernel-team@...roid.com>,
Boqun Feng <boqun.feng@...il.com>,
Byungchul Park <byungchul.park@....com>,
Ingo Molnar <mingo@...hat.com>,
Masami Hiramatsu <mhiramat@...nel.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Namhyung Kim <namhyung@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Thomas Glexiner <tglx@...utronix.de>,
Tom Zanussi <tom.zanussi@...ux.intel.com>
Subject: Re: [PATCH v12 3/3] tracing: Centralize preemptirq tracepoints and
unify their usage
On Wed, 8 Aug 2018 06:03:02 -0700
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> What's wrong with a this_cpu_inc()? It's atomic for the CPU. Although
> > it wont be atomic for the capture of the idx. But I also don't see
> > interrupts being disabled, thus an NMI is no different than any
> > interrupt doing the same thing, right?
>
> On architectures without increment-memory instructions, if you take an NMI
> between the load from sp->sda->srcu_lock_count and the later store, you
> lose a count. Note that both __srcu_read_lock() and __srcu_read_unlock()
> do increments of different locations, so you cannot rely on the usual
> "NMI fixes up before exit" semantics you get when incrementing and
> decrementing the same location.
And how is this handled in the interrupt case? Interrupts are not
disabled here.
I would also argue that architectures without increment-memory
instructions shouldn't have NMIs ;-)
-- Steve
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