Jan Kiszka wrote:
> Gilles Chanteperdrix wrote:
>> Jan Kiszka wrote:
>>> Gilles Chanteperdrix wrote:
>>>> Gilles Chanteperdrix wrote:
>>>>> Jan Kiszka wrote:
>>>>>> Gilles Chanteperdrix wrote:
>>>>>>> Jan Kiszka wrote:
>>>>>>>> Gilles Chanteperdrix wrote:
>>>>>>>>> Jan Kiszka wrote:
>>>>>>>>>> Gilles Chanteperdrix wrote:
>>>>>>>>>>> Jan Kiszka wrote:
>>>>>>>>>>>> Gilles Chanteperdrix wrote:
>>>>>>>>>>>>> Jan Kiszka wrote:
>>>>>>>>>>>>>> + xnarch_atomic_set(mutex->owner,
>>>>>>>>>>>>>> + set_claimed(xnthread_handle(owner),
>>>>>>>>>>>>>> +
>>>>>>>>>>>>>> xnsynch_nsleepers(&mutex->synchbase)));
>>>>>>>>>>>>> Ok. I think you have spotted a bug here. This should be
>>>>>>>>>>>>> mutex->sleepers
>>>>>>>>>>>>> instead of xnsynch_nsleepers.
>>>>>>>>>>>> BTW, why do you need to track sleepers separately in POSIX? Native
>>>>>>>>>>>> doesn't do so, e.g.
>>>>>>>>>>> Because of the "syscall-needed-when-unlocking-stolen-mutex" issue I
>>>>>>>>>>> already explained (sleepers - xnsynch_nsleepers is precisely the
>>>>>>>>>>> count
>>>>>>>>>>> of pending threads which have been awake then robbed the mutex).
>>>>>>>>>> Hmm, sounds like the new lock owner should better clear the 'claimed'
>>>>>>>>>> bit then, not the old one on return from unlock. Or where is the
>>>>>>>>>> pitfall? How does the futex algorithm handle this scenario?
>>>>>>>>> Ok. Please read my explanation again, I have already explained this in
>>>>>>>>> another mail.
>>>>>>>> I did this, but I'm unable to derive the answer for my question from
>>>>>>>> it.
>>>>>>>> Let's go through it in more details:
>>>>>>>>
>>>>>>>> When we pass a mutex to a new owner, we set its reference in the fast
>>>>>>>> lock variable + set the claimed bit if there are more waiters. Instead,
>>>>>>>> I would simple set that bit if there is a new owner. That owner will
>>>>>>>> then pick up the mutex eventually and clear 'claimed' on exit from it
>>>>>>>> lock service (if there are no further waiters then). If the new owner
>>>>>>>> is
>>>>>>>> not able to run and we steal the lock, we simple keep the 'claimed' bit
>>>>>>>> as is. On exit from the stolen lock we find it set, thus we are forced
>>>>>>>> to issue a syscall as it should be.
>>>>>>>>
>>>>>>>> OK, what happens if some waiter wants to leave the party while we are
>>>>>>>> holding the stolen lock? Then the sleeper number must be correct - that
>>>>>>>> is one pitfall!
>>>>>>>>
>>>>>>>> I will have to dig into this more deeply, considering more cases. But
>>>>>>>> the additional "sleepers" field remains at least misplaced IMHO.
>>>>>>>> xnsynch_sleepers should better be fixed to respect lock stealing, as
>>>>>>>> lock stealing is an xnsynch property, nothing POSIX-specific.
>>>>>>> Ok. I have read this but did not get what you mean. I will read it again
>>>>>>> quietly from home.
>>>>>> 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".
>>>> By the way, I think we should stop sending mails to our personal
>>>> addresses in addition to the mailing list, because this results in
>>>> mailing list mails being received out of orders, which make the threads
>>>> hard to follow.
>>> That's common practice on most mailing lists I know of, and I personally
>>> don't want to change this. IMO, it would only make replying more
>>> complicated, and it would bear the risk to drop CCs to non-subscribers.
>>>
>>> I think the problem was only temporarily, maybe caused by some weird
>>> interaction of gna.org and the Siemens mailserver (we were too fast for
>>> them). Meanwhile, archives and inboxes should contain all messages.
>>> Gmane, e.g., lists them in the correct order now.
>> The "weird interaction" is actually a feature and called "grey listing".
>> It delays mails for at least 5 minutes, the real result depending on the
>> eagerness of your relay mail server to re-transmit the delayed message.
>
> I'm aware of such anti-spam approaches (and their varying effect). But
> as I've seen specifically gna.org struggling with other mailservers as
> well, just recently, I'm careful with blaming solely our corporate
> server. Logs would help to clarify, but I don't have access to either side.
Here is an exerpt of the logs:
2008-08-27 16:55:54 1KYMR8-0004Uw-2w SMTP error from remote mailer after
initial connection: host mail.gna.org [88.191.250.46]: 421 mail.gna.org
service not available, please try later.
Then:
2008-08-27 16:55:55 1KYMR8-0004Uw-2w => [EMAIL PROTECTED]
R=dnslookup T=remote_smtp H=hephaistos.siemens.com [217.194.34.77]
X=TLS-1.0:RSA_AES_256_CBC_SHA:32
2
And finally:
2008-08-27 17:13:08 1KYMR8-0004Uw-2w => xenomai-core@gna.org R=dnslookup
T=remote_smtp H=mail.gna.org [88.191.250.46]*
2008-08-27 17:13:08 1KYMR8-0004Uw-2w Completed
--
Gilles.
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