linux-kernel - Re: [RFC][PATCH] x86: make text

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Date:	Mon, 2 Mar 2009 10:42:21 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Arjan van de Ven <arjan@...radead.org>
cc:	Masami Hiramatsu <mhiramat@...hat.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"H. Peter Anvin" <hpa@...or.com>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [RFC][PATCH] x86: make text_poke() atomic



On Mon, 2 Mar 2009, Arjan van de Ven wrote:
> 
> .... but text_poke() realistically needs to call stop_machine() since
> you can't poke live code.... so that makes me wonder how useful this
> is...

Well, not always. There's at least two cases where we don't need it:

 - in the UP -> SMP transition.

 - perhaps more interestingly, we're still kind of waiting for the 
   resolution of the whole "nop out the first byte to a single-byte 'irq3' 
   trap instruction, then rewrite the rest of the instruction, and then 
   reset the first byte of the final instruction" thing.

IOW, there are possible non-stop_machine() models where rewriting 
instructions in a live system does work, and quite frankly, I think we 
need them. Making the rule be the (obviously safe) "we can only do this in 
stop_machine" is quite possibly not going to be an acceptable rule and we 
may need alternatives.

			Linus
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