On 3/2/20 6:12 PM, Petr Mladek wrote:

On Mon 2020-03-02 16:45:24, JeffleXu wrote:

quoted

Hello guys,


I'm new to livepatch world and now I'm a little confused with livepatch's

per-task consistency model which is introduced by [1]. I've also readed the

discussion on mailing list [2], which introduces shadow variable to handle

data layout and semantic changes. But there's still some confusion with this

per-task consistency model.


According to the model, there will be scenario where old function and new

function can co-exist, though for a single thread, it sees either all new

functions or all old functions.


I can't understand why Vojtech said that 'old func processing new data' was

impossible. Assuming a scenario where a process calls func-A to submit a

work request (inserted into a global list), and then a kthread is
responsible

for calling func-B to process all work requests in the list. What if this
process

has finished the transition (sees new func-A) while kthread still sees the
old func-B?

In this case, old func-B has to process new data. If there's some lock
semantic

changes in func-A and func-B, then old func-B has no way identifying the
shadow

variable labeled by new func-A.


Please tell me if I missed something, and any suggestions will be
appreciated. ;)

No, you did not miss anything. The consistency is only per-thread,
If a livepatch is changing semantic of a global variable it must
allow doing the changes only when the entire system is using
the new code. And the new code must be able to deal with both
old and new data.

The new data semantic can be enable by post-patch callback
that is called when the transition has finished.

Thanks for replying. I didn't consider callback earlier, and yes it 
works in this case.


Thanks.

Jeffle