Subject: Re: barriers: was: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model

From: Petr Mladek <[email-masked]>
Date: Wed, 4 May 2016 16:12:05 +0200

On Wed 2016-05-04 14:39:40, Petr Mladek wrote:
> 		 *
> 		 * Note that the task must never be migrated to the target
> 		 * state when being inside this ftrace handler.
> 		 */
> 
> We might want to move the second paragraph on top of the function.
> It is a basic and important fact. It actually explains why the first
> read barrier is not needed when the patch is being disabled.

I wrote the statement partly intuitively. I think that it is really
somehow important. And I am slightly in doubts if we are on the safe side.

First, why is it important that the task->patch_state is not switched
when being inside the ftrace handler?

If we are inside the handler, we are kind-of inside the called
function. And the basic idea of this consistency model is that
we must not switch a task when it is inside a patched function.
This is normally decided by the stack.

The handler is a bit special because it is called right before the
function. If it was the only patched function on the stack, it would
not matter if we choose the new or old code. Both decisions would
be safe for the moment.

The fun starts when the function calls another patched function.
The other patched function must be called consistently with
the first one. If the first function was from the patch,
the other must be from the patch as well and vice versa.

This is why we must not switch task->patch_state dangerously
when being inside the ftrace handler.

Now I am not sure if this condition is fulfilled. The ftrace handler
is called as the very first instruction of the function. Does not
it break the stack validity? Could we sleep inside the ftrace
handler? Will the patched function be detected on the stack?

Or is my brain already too far in the fantasy world?

Best regards,
Petr