Re: [PATCH v3 00/15] livepatch: hybrid consistency model

Balbir Singh
2016-12-11T02:08:33Z

On 11/12/16 04:17, Josh Poimboeuf wrote:
> On Sat, Dec 10, 2016 at 04:46:17PM +1100, Balbir Singh wrote:
>> On Thu, 2016-12-08 at 12:08 -0600, Josh Poimboeuf wrote:
>>> Dusting the cobwebs off the consistency model again. This is based on
>>> linux-next/master.
>>>
>>> v1 was posted on 2015-02-09:
>>>
>>> https://lkml.kernel.org/r/cover.1423499826.git.jpoimboe@redhat.com
>>>
>>> v2 was posted on 2016-04-28:
>>>
>>> https://lkml.kernel.org/r/cover.1461875890.git.jpoimboe@redhat.com
>>>
>>> The biggest issue from v2 was finding a decent way to detect preemption
>>> and page faults on the stack of a sleeping task.
>>
>> Could you please elaborate on this? Preemption of a sleeping task and
>> faults as in the future (time) preemption and faults?
>
> The normal way for a task to go to sleep is to call schedule(). objtool
> ensures the stack trace is reliable in that case, by making sure that
> all functions save the frame pointer on the stack before calling out to
> another function.
>
> But a task can also go to sleep in a few other ways. One way is by
> preemption, where an interrupt handler interrupts the task and calls
> preempt_schedule_irq().

It's preempted, not sleeping. It's on_rq but not on_cpu.

Another way is by a page fault exception. In
> both cases, there's no guarantee that the interrupted function saved the
> frame pointer on the stack beforehand. So the stack trace might be
> unreliable. Fortunately, interrupts and exceptions leave evidence
> behind on the stack. So when walking the stack of a sleeping task, we
> can detect when an IRQ or exception occurred, and consider such a stack
> unreliable.
>

Thanks for the explanation. I presume a whole lot of this is arch specific
code? I'll look at the patches as well

Balbir