linux-kernel - Re: [PATCH 0/3] ring-buffer: less locking and only disable preemption

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Date:	Sun, 5 Oct 2008 12:13:41 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [PATCH 0/3] ring-buffer: less locking and only disable
	preemption

* Mathieu Desnoyers <compudj@...stal.dyndns.org> wrote:

> explains that code modification on x86 SMP systems is not only a 
> matter of atomicity, but also a matter of not changing the code 
> underneath a running CPU which is making assumptions that it won't 
> change underneath without issuing a synchronizing instruction before 
> the new code is used by the CPU. The scheme you propose here takes 
> care of atomicity, but does not take care of the synchronization 
> problem. A sync_core() would probably be required when such 
> modification is detected.

that's wrong, my scheme protects against these cases: before _any_ code 
is modified we set the redo_pending atomic flag, and make sure that 
previous NMI handlers have stopped executing. (easy enough)

then the atomic update of redo_pending should be a sufficient barrier 
for another CPU to notice the pending transaction.

Note that the cross-CPU modification can still be 'half done' when the 
NMI hits, that's why we execute modify_code_redo() to 'redo' the full 
modification before executing further NMI code. That is executed _on the 
CPU_ that triggers an NMI, and the CPU itself is self-consistent.

( The modify_code_redo() will have to do a sync_cores() of course, like 
  all self-modifying code, to flush speculative execution. )

> Also, speaking of plain atomicity, you scheme does not seem to protect 
> against NMIs running on a different CPU, because the non-atomic change 
> could race with such NMI.

That's wrong too. Another CPU will notice that redo_pending is set and 
will execute modify_code_redo() from its NMI handler _before_ calling 
all the notifiers and other 'wide' code paths.

the only item that needs to be marked 'notrace' is only the highlevel 
do_nmi() handler itself. (as that executes before we have a chance to 
execute modify_code_redo())

So we trade a large, fragile, and unmapped set of NMI-implicated 
codepaths for a very tight and well controlled an easy to maintain 
codepath.

	Ingo
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