Re: [Xen-devel] Issue policing writes from Xen to PV domain memory

["Aravindh Puthiyaparambil (aravindp)" <aravindp@xxxxxxxxx>]

>>>That would effectively be a spin wait then, which is surely not the right
>thing.
>>>But I don't follow your explanation above anyway - when coming back
>>>from wait(), the state is the same as the original one, so the page
>>>fault handling continues, it's not being retried.
>>
>> Let me step through this. The pagefault for runstate occurs and
>> wait_event() gets called and the ring is full.
>>
>> wait_event():
>> do {
>>     if ( mem_event_wait_try_grab(med, &rc) != -EBUSY )       ==> This will
>return EBUSY
>>         break;
>>     for ( ; ; ) {
>>         prepare_to_wait(&med->wq);
>>         if ( mem_event_wait_try_grab(med, &rc) != -EBUSY )   ==> This will
>return EBUSY
>>             break;
>>         wait();                                                              
>>                                      ==> Write to
>runstate area occurs...
>>     }
>>     finish_wait(&med->wq);
>> } while (0)
>>
>> ...now this write to runstate will again cause a pagefault and we will
>> end up in wait_event() again. The previous attempt would have called
>> prepare_to_wait() but finish_wait() was not called.
>> finish_wait()->__finish_wait() is where wqv->esp gets reset to NULL. So
>now:
>>
>> wait_event():
>> do {
>>     if ( mem_event_wait_try_grab(med, &rc) != -EBUSY )       ==> This will
>return EBUSY
>>         break;
>>     for ( ; ; ) {
>>         prepare_to_wait(&med->wq);                                           
>>       ==> Assertion
>'wqv->esp == 0' fails
>>         if ( mem_event_wait_try_grab(med, &rc) != -EBUSY )
>>             break;
>>         wait();
>>     }
>>     finish_wait(&med->wq);
>> } while (0)
>>
>> Does this make sense?
>
>Of course. But it still doesn't mean you can replace the wait() here with
>something else (e.g. the effectively spin wait loop you suggested earlier). As
>said earlier on, you need to find a way to avoid the second invocation of
>wait_event(), rather than a way to make it "work".

OK, I will look in to that. Thanks for your time.

Aravindh


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