Re: [PATCH v3 5/6] x86/alternative: Use a single access in text_poke() where possible

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Fri, Jan 11, 2019 at 05:46:36PM +0100, Alexandre Chartre wrote:
> 
> 
> On 01/11/2019 04:28 PM, Josh Poimboeuf wrote:
> > On Fri, Jan 11, 2019 at 01:10:52PM +0100, Alexandre Chartre wrote:
> > > To avoid any issue with live patching the call instruction, what about
> > > toggling between two call instructions: one would be the currently active
> > > call, while the other would currently be inactive but to be used after a
> > > change is made. You can safely patch the inactive call and then toggle
> > > the call flow (using a jump label) between the active and inactive calls.
> > > 
> > > So instead of having a single call instruction:
> > > 
> > > 	call function
> > > 
> > > You would have:
> > > 
> > > 	STATIC_JUMP_IF_TRUE label, key
> > > 	call function1
> > > 	jmp done
> > > label:
> > > 	call function2
> > > done:
> > > 
> > > If the key is set so that function1 is currently called then you can
> > > safely update the call instruction for function2. Once this is done,
> > > just flip the key to make the function2 call active. On a next update,
> > > you would, of course, have to switch and update the call for function1.
> > 
> > What about the following race?
> > 
> > CPU1						CPU2
> > static key is false, doesn't jump
> > task gets preempted before calling function1
> > 						change static key to true
> > 						start patching "call function1"
> > task resumes, sees inconsistent call instruction
> > 
> 
> If the function1 call is active then it won't be changed, you will change
> function2. However, I presume you can still have a race but if the function
> is changed twice before calling function1:
> 
> CPU1						CPU2
> static key is false, doesn't jump
> task gets preempted before calling function1
>                                                 -- first function change --
>                                                 patch "call function2"
>                                                 change static key to true
>                                                 -- second function change --
>                                                 start patching "call function1"
> task resumes, sees inconsistent call instruction
> 
> So right, that's a problem.

Right, that's what I meant to say :-)

-- 
Josh