Re: [RFC PATCH 2/6] jump label v3 - x86: Introduce generic jump patching without stop_machine

["H. Peter Anvin" <hpa@zytor.com>]

On 11/18/2009 02:43 PM, Jason Baron wrote:
> Add text_poke_fixup() which takes a fixup address to where a processor
> jumps if it hits the modifying address while code modifying.
> text_poke_fixup() does following steps for this purpose.
> 
>  1. Setup int3 handler for fixup.
>  2. Put a breakpoint (int3) on the first byte of modifying region,
>     and synchronize code on all CPUs.
>  3. Modify other bytes of modifying region, and synchronize code on all CPUs.
>  4. Modify the first byte of modifying region, and synchronize code
>     on all CPUs.
>  5. Clear int3 handler.
> 
> Thus, if some other processor execute modifying address when step2 to step4,
> it will be jumped to fixup code.
> 
> This still has many limitations for modifying multi-instructions at once.
> However, it is enough for 'a 5 bytes nop replacing with a jump' patching,
> because;
>  - Replaced instruction is just one instruction, which is executed atomically.
>  - Replacing instruction is a jump, so we can set fixup address where the jump
>    goes to.
> 

I just had a thought about this... regardless of if this is safe or not
(which still remains to be determined)... I have a bit more of a
fundamental question about it:

This code ends up taking *two* global IPIs for each instruction
modification.  Each of those requires whole-system synchronization.  How
is this better than taking one IPI and having the other CPUs wait until
the modification is complete before returning?

	-hpa