linux-kernel - Re: [PATCH 1/5] tracing: convert ftrace

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Date:	Wed, 29 Apr 2009 07:55:12 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 1/5] tracing: convert ftrace_dump spinlocks to raw

* Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Wed, 29 Apr 2009 00:48:15 -0400 Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > +	local_irq_save(flags);
> > +	__raw_spin_lock(&ftrace_dump_lock);
> 
> I'm counting twentyish instances of this. [...]

there's 9 in the tracing code, two more in the rest of the kernel.

> [...] Maybe it's time to give up and implement 
> raw_spin_lock_irqsave().

There's a difference between:

	local_irq_save(flags);
	__raw_spin_lock(&ftrace_dump_lock);

and:

	raw_local_irq_save(flags);
	__raw_spin_lock(&ftrace_dump_lock);

raw_spin_lock_irqsave() would map to the latter logic-wise - but in 
the tracing code we want the former.

But the main reason is that using raw locks has its costs - it skips 
lockdep coverage, is preempt unsafe, etc. The APIs are not complete 
and intentionally so: they are minimal building blocks for the real 
APIs. Instrumentation goes deep in the guts of the kernel so using 
very low level primitives is justified there - but we dont want to 
make it _too_ easy to use a raw-anything locking facility.

	Ingo
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