linux-kernel - Re: [GIT PULL] tracing: make signal tracepoints more useful

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Date:	Tue, 17 Jan 2012 07:03:31 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Seiji Aguchi <saguchi@...hat.com>,
	linux-kernel@...r.kernel.org,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Subject: Re: [GIT PULL] tracing: make signal tracepoints more useful

On Tue, 2012-01-17 at 11:02 +0100, Ingo Molnar wrote:

> That is not true *AT ALL* in such an unqualified manner. Steve, 
> stop being stupid.
> 
> The kernel syscall ABI may indeed sometimes expand *INPUT* 
> structures (if via some mechanism it's possible to make sure 
> that old ABI uses don't cause the kernel to read undefined 
> data), but the trace events are *OUTPUT* structures.

The difference between syscalls and tracepoints is that a tracepoint
always reports the size of the structure that was read, where a syscall
does not. So I do consider this similar to reading the /proc/stat file
as the user can see how much was read. The backwards compatibility
should be easy to write. Old tools should not break, because it wont be
reading the new fields, and new tools can determine which tracepoint is
there because it is trivial to see which version of the tracepoint is
there because of the size read.

But as I'm stupid, I'll shut up now.

-- Steve

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