linux-kernel - Re: [PATCH] tracing: use raw spinlocks instead of spinlocks

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Message-ID: <c62985530811021717w169111dah4574efaf124707c2@mail.gmail.com>
Date:	Mon, 3 Nov 2008 02:17:35 +0100
From:	"Frédéric Weisbecker" <fweisbec@...il.com>
To:	"Steven Rostedt" <rostedt@...dmis.org>
Cc:	"Ingo Molnar" <mingo@...e.hu>,
	"Linux Kernel" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tracing: use raw spinlocks instead of spinlocks

2008/11/3 Steven Rostedt <rostedt@...dmis.org>:
> Frederic,
>
> Ingo has been very adamant about not using raw_spin_locks in the ring
> buffers. My original code did this, and he nacked it. The reason being
> (and he eventually convinced me) was that by using raw, we not only do not
> trace the locking, we also remove the lock checking. This code can easily
> produce deadlocks, so we do not want the lock checking removed.

Ok. I understand...

> The real fix is to find a way in your tracer to detect the recursion, and
> be able to prevent it. Like the atomic disables I use in ftrace. It does
> the same thing. It leaves the lockdep checking on its own locks, but can
> also detect if the lock checking caused it to recurse. When the recusion
> is detected, the tracer itself will not trace.

You mean this part in function_trace_call?

disabled = atomic_inc_return(&data->disabled);
if (likely(disabled == 1))
		trace_function(tr, data, ip, parent_ip, flags, pc);

atomic_dec(&data->disabled);

That's a good idea. Since it applies on one cpu_data specific and
preempt and irq are disabled, the function
tracer doesn't risk to loose a trace. I think I will apply the same method.

Thanks!

> [ I hope this makes sense, I'm writing this on 3 hours of sleep ]

Yes, don't worry :-)
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