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Date: Mon, 2 Mar 2009 10:42:21 -0800 (PST)
From: Linus Torvalds <torvalds@linux-foundation.org>
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To: Arjan van de Ven <arjan@infradead.org>
cc: Masami Hiramatsu <mhiramat@redhat.com>,
       Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
       Ingo Molnar <mingo@elte.hu>, Andrew Morton <akpm@linux-foundation.org>,
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       Rusty Russell <rusty@rustcorp.com.au>, "H. Peter Anvin" <hpa@zytor.com>,
       Steven Rostedt <srostedt@redhat.com>
Subject: Re: [RFC][PATCH] x86: make text_poke() atomic
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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