linux-kernel - Re: [for-next][PATCH 1/4] ring-buffer: Replace this_cpu_*() with __this_cpu

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Message-ID: <alpine.DEB.2.11.1503271423580.23114@gentwo.org>
Date:	Fri, 27 Mar 2015 14:41:44 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	stable@...r.kernel.org,
	Uwe Kleine-Koenig <u.kleine-koenig@...gutronix.de>
Subject: Re: [for-next][PATCH 1/4] ring-buffer: Replace this_cpu_*() with
 __this_cpu_*()

On Wed, 25 Mar 2015, Steven Rostedt wrote:

> It has come to my attention that this_cpu_read/write are horrible on
> architectures other than x86. Worse yet, they actually disable
> preemption or interrupts! This caused some unexpected tracing results
> on ARM.

This isnt something new and I thought the comment was dropped from the
patch? This is a plain error in using this_cpu_* where __this_cpu_* would
have been sufficient. Code was uselessly disabling preemption twice.

> Which is unacceptable for locations that know they are within preempt
> disabled or interrupt disabled locations.

Well yes. Thats why the __this_cpu ops are there to avoid this
overhead.

> I also changed the recursive_unlock() to use two local variables instead
> of accessing the per_cpu variable twice.

Ok gotta look at that.

>  static __always_inline void trace_recursive_unlock(void)
>  {
> -	unsigned int val = this_cpu_read(current_context);
> +	unsigned int val = __this_cpu_read(current_context);
>
> -	val--;
> -	val &= this_cpu_read(current_context);
> -	this_cpu_write(current_context, val);
> +	val &= val & (val - 1);
> +	__this_cpu_write(current_context, val);
>  }

Ummm... This is does not look like an equivalent thing. Should this not
be:

	unsigned int val = __this_cpu_read(current_context);
	unsigned int newval = val - 1;

	newval &= val;
	__this_cpu_write(current_context, newval);

or more compact

	unsigned int val = __this_cpu_read(current_context);

	__this_cpu_write(current_context, val & (val - 1));

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