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.
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