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

[Balbir Singh]

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@xxxxxxxxxx
>>>  
>>> v2 was posted on 2016-04-28:
>>>  
>>>   https://lkml.kernel.org/r/cover.1461875890.git.jpoimboe@xxxxxxxxxx
>>>  
>>> 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