linux-kernel - Re: [PATCH v12 3/3] tracing: Centralize preemptirq tracepoints and unify their usage

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Date:   Wed, 8 Aug 2018 06:03:02 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@...dmis.org>
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, Aug 08, 2018 at 08:46:29AM -0400, Steven Rostedt wrote:
> On Tue, 7 Aug 2018 20:53:54 -0700
> Joel Fernandes <joelaf@...gle.com> wrote:
> 
> 
> > > 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?
> > 
> > It does start to seem like a show stopper :-(
> 
> 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.

							Thanx, Paul