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

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Date:   Wed, 8 Aug 2018 09:03:50 -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 11:27:05AM -0400, Steven Rostedt wrote:
> On Wed, 8 Aug 2018 07:42:00 -0700
> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > > There's also a local_inc() if you are using per cpu pointers, that is
> > > suppose to guarantee atomicity for single cpu operations. That's what
> > > the ftrace ring buffer uses.  
> > 
> > Good point, that becomes atomic_long_inc() or equivalent on most
> > architectures, but an incl instruction (not locked) on x86.  So updating
> > my earlier still-untested thought:
> > 
> > int __srcu_read_lock_nmi(struct srcu_struct *sp)  /* UNTESTED. */
> > {
> > 	int idx;
> > 
> > 	idx = READ_ONCE(sp->srcu_idx) & 0x1;
> > 	local_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. */
> > 	local_inc(&sp->sda->srcu_unlock_count[idx]);
> > }
> > 
> > Would that work, or is there a better way to handle this?
> 
> This would work much better than using the atomic ops, and I think it
> would be doable.

OK, here is hoping!

> I may need to revert the srcu for trace_*_rcidle() for now, as I want
> most of the other changes in this merge window, and it's getting too
> late to do it with these updates.

Agreed, especially since I normally freeze for the next merge window
shortly after -rc5.  ;-)

							Thanx, Paul