Re: [RFC] [PATCH 1/2 v2] x86: introduce int3-based instruction patching

[Steven Rostedt <rostedt@goodmis.org>]

On Wed, 2013-07-10 at 14:36 -0700, H. Peter Anvin wrote:
> On 07/10/2013 02:31 PM, Jiri Kosina wrote:
> > 
> > If any CPU instruction execution would collide with the patching,
> > it'd be trapped by the int3 breakpoint and redirected to the provided
> > "handler" (which would typically mean just skipping over the patched
> > region, acting as "nop" has been there, in case we are doing nop -> jump
> > and jump -> nop transitions).
> > 
> 
> I'm wondering if it would be easier/more general to just return to the
> instruction.  The "more general" bit would allow this to be used for
> other things, like alternatives, and perhaps eventually dynamic call
> patching.
> 
> Returning to the instruction will, in effect, be a busy-wait for the
> faulted CPU until the patch is complete; more or less what stop_machine
> would do, but only for a CPU which actually strays into the affected region.
> 

Wont work for ftrace, as it patches all functions, it even patches
functions used to do the changes. Thus, it would cause a deadlock if a
breakpoint were to spin till the changes were finished.

-- Steve