Gilles Chanteperdrix wrote:
> Jan Kiszka wrote:
>> Gilles Chanteperdrix wrote:
>>> Philippe Gerum wrote:
>>>> Gilles Chanteperdrix wrote:
>>>>> Jan Kiszka wrote:
>>>>>> Gilles Chanteperdrix wrote:
>>>>>>> Jan Kiszka wrote:
>>>>>> ...
>>>>>>>> I think I'm getting closer to the issue. Our actual problem comes from
>>>>>>>> the fact that the xnsynch_owner is easily out of sync with the real
>>>>>>>> owner, it even sometimes points to a former owner:
>>>>>>>>
>>>>>>>> Thread A releases a mutex on which thread B pends. It wakes up B,
>>>>>>>> causing it to become the new xnsynch owner, and clears the claimed bit
>>>>>>>> as there are no further sleepers. B returns, and when it wants to
>>>>>>>> release the mutex, it does this happily in user space because claimed 
>>>>>>>> is
>>>>>>>> not set. Now the fast lock variable is 'unlocked', while xnsynch still
>>>>>>>> reports B being the owner. This is no problem as the next time two
>>>>>>>> threads fight over this lock the waiter will simply overwrite the
>>>>>>>> xnsynch_owner before it falls asleep. But this "trick" doesn't work for
>>>>>>>> waiters that have been robbed. They will spin inside xnsynch_sleep_on
>>>>>>>> and stumble over this inconsistency.
>>>>>>>>
>>>>>>>> I have two approaches in mind now: First one is something like
>>>>>>>> XNSYNCH_STEALNOINFORM, i.e. causing xnsynch_sleep_on to not set 
>>>>>>>> XNROBBED
>>>>>>>> so that the robbed thread spins one level higher in the skin code -
>>>>>>>> which would have to be extended a bit.
>>>>>>> No, the stealing is the xnsynch job.
>>>>>>>
>>>>>>>> Option two is to clear xnsynch_owner once a new owner is about to 
>>>>>>>> return
>>>>>>>> from kernel with the lock held while there are no more 
>>>>>>>> xnsynch_sleepers.
>>>>>>>> That should work with even less changes and save us one syscall in the
>>>>>>>> robbed case. Need to think about it more, though.
>>>>>>> In fact the only time when the owner is required to be in sync is when
>>>>>>> PIP occurs, and this is guaranteed to work, because when PIP is needed a
>>>>>>> syscall is emitted anyway. To the extent that xnsynch does not even
>>>>>>> track the owner on non PIP synch (which is why the posix skin originally
>>>>>>>  forcibly set the synch owner, and it was simply kept to get the fastsem
>>>>>>> stuff working).
>>>>>>>
>>>>>>> Ok. And what about the idea of the xnsynch bit to tell him "hey, the
>>>>>>> owner is tracked in the upper layer, go there to find it".
>>>>>> I'm yet having difficulties to imagine how this should look like when
>>>>>> it's implemented. Would it be simpler than my second idea?
>>>>>>
>>>>>> Anyway, here is a patch (on top of my handle-based lock series) for the
>>>>>> approach that clears xnsynch_owner when there are no waiters. At least
>>>>>> it causes no regression based on your test, but I haven't checked lock
>>>>>> stealing yet. In theory, everything still appears to be fine to me. This
>>>>>> approach basically restores the state we find when some thread just
>>>>>> acquired the lock in user space.
>>>>> Yes, I did not think about the stealing when writing my test, but I
>>>>> think it could be a good idea to add it to the test, especially if you
>>>>> want to port the test to the native API.
>>>>>
>>>>> I let Philippe decide here. He is the one who did the stealing stuff and
>>>>> probably knows better.
>>>>>
>>>> Currently, the xnsynch strongly couples PIP and ownership, which seems to 
>>>> impede
>>>> your various proposals. I would suggest to decouple that: the basic 
>>>> property of
>>>> some xnsynch that we may want to handle is exclusiveness, then dynamic 
>>>> priority
>>>> inheritance is another property, that could stack its own semantics on top 
>>>> of
>>>> exclusiveness.
>>>>
>>>> XNSYNCH_EXCLUSIVE would cover all ownership-related actions, XNSYNCH_PIP 
>>>> would
>>>> simply add dynamic priority management. Non exclusive object would not 
>>>> require
>>>> any xnsynch_set_owner() handling.
>>>>
>>>> Just to give a clear signal here: I will happily consider any change to the
>>>> xnsynch object that may ease the implementation of fast ownership handling 
>>>> (i.e.
>>>> userland-to-userland transfer). The only thing is that such code is very 
>>>> much
>>>> prone to regressions, so a testsuite must come with core changes in that 
>>>> area,
>>>> but I guess you know that already.
>>> Ok. I think unit_mutex.c is a good start. It only lacks testing
>>> XNROBBED.
>> My colleague sent me an extension. It's native-only so far, but it
>> already pointed out a bug in my try-acquire implementation that should
>> be present in posix as well (trylock must go through the slow path).
> 
> I do not see why. If mutex lock can lock without a syscall, the same
> goes for trylock.

Lock stealing requires the slow path.

Jan


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