linux-kernel - Re: [PATCH v2 1/1] ftrace: do not update max buffer with no users

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Message-ID: <alpine.DEB.1.10.0811130848450.7458@gandalf.stny.rr.com>
Date:	Thu, 13 Nov 2008 08:54:58 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Pekka Paalanen <pq@....fi>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH v2 1/1] ftrace: do not update max buffer with no users


On Thu, 13 Nov 2008, Ingo Molnar wrote:
> 
> the obvious solution is to add this to ring_buffer_resize():
> 
> 	if (!buffer)
> 		return size;

Having a NULL buffer return a successful resize is a bit worrisome to me.

And looking at the code I was trying to make sure could never be called
if there are no max_tr users:

in update_max_tr

	buf = tr->buffer;
	tr->buffer = max_tr.buffer;
	max_tr.buffer = buf;

Should all the ring buffer API return success on NULL pointers?

> 
> resizing a non-existent buffer should succeed. A two-liner patch. Not 
> 160 lines of flux.
> 
> Really, you need to think _hard_ how to avoid invasive-looking changes 
> in late -rc's, because every extra line to review uses up precious 
> review resources.

I appreciate the goal of minimal change, but I also want to keep things 
robust.

Right now I'm thinking my other suggestion is the best. Just allocate the 
max_tr.buffer and get rid of the CONFIG. This solution is very small, and 
covers all corner cases.

-- Steve

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