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SubjectRe: [PATCH v2] tracing: Make sure RCU is watching before calling a stack trace
On Thu, 18 May 2017 15:48:55 +0200 (CEST)
Miroslav Benes <mbenes@suse.cz> wrote:

> On Thu, 18 May 2017, Steven Rostedt wrote:
>
> >
> > From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> >
> > As stack tracing now requires "rcu watching", force RCU to be watching when
> > recording a stack trace.
> >
> > Link: http://lkml.kernel.org/r/20170512172449.879684501@goodmis.org
> >
> > Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> > Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> > ---
> >
> > Changes since v1:
> >
> > My testing discovered that the stack trace can be called with
> > interrupts enabled, which is a no no to have when calling
> > rcu_irq_enter(). When interrupts are enabled, as with being in an
> > NMI, RCU will also be watching.
>
> Would rcu_irq_enter_irqson() help then? This is what Petr used in a live
> patching handler.
>

Yes, that could work too, but I wanted to avoid disabling interrupts if
we didn't have to.

> Your solution works too, of course. Just asking if I am not missing
> something.
>

Nope, I was just trying to keep the overhead down. As this can be
called by every event enabled, as well as functions being traced. I
figured that local_save_irqs() is faster than a pair of
local_irq_save()/ local_irq_restore() calls.

-- Steve

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