On 05/31/2011 07:06 PM, Jan Kiszka wrote:
> On 2011-05-31 18:58, Philippe Gerum wrote:
>> On Tue, 2011-05-31 at 18:38 +0200, Jan Kiszka wrote:
>>> On 2011-05-31 18:29, Philippe Gerum wrote:
>>>> On Tue, 2011-05-31 at 13:37 +0200, Jan Kiszka wrote:
>>>>> Hi Philippe,
>>>>>
>>>>> enabling XENO_OPT_DEBUG_NUCLEUS reveals some shortcomings of the
>>>>> in-kernel lock usage tracking via xnthread_t::hrescnt. This BUGON in
>>>>> xnsynch_release triggers for RT threads:
>>>>>
>>>>>   XENO_BUGON(NUCLEUS, xnthread_get_rescnt(lastowner) < 0);
>>>>>
>>>>> RT threads do not balance their lock and unlock syscalls, so their
>>>>> counter goes wild quite quickly.
>>>>>
>>>>> But just limiting the bug check to XNOTHER threads is neither a
>>>>> solution. How to deal with the counter on scheduling policy changes?
>>>>>
>>>>> So my suggestion is to convert the auto-relax feature into a service,
>>>>> user space can request based on a counter that user space maintains
>>>>> independently. I.e. we should create another shared word that user space
>>>>> increments and decrements on lock acquisitions/releases on its own. The
>>>>> nucleus just tests it when deciding about the relax on return to user 
>>>>> space.
>>>>>
>>>>> But before hacking into that direction, I'd like to hear if it makes
>>>>> sense to you.
>>>>
>>>> At first glance, this does not seem to address the root issue. The
>>>> bottom line is that we should not have any thread release an owned lock
>>>> it does not hold, kthread or not.
>>>>
>>>> In that respect, xnsynch_release() looks fishy because it may be called
>>>> over a context which is _not_ the lock owner, but the thread who is
>>>> deleting the lock owner, so assuming lastowner == current_thread when
>>>> releasing is wrong.
>>>>
>>>> At the very least, the following patch would prevent
>>>> xnsynch_release_all_ownerships() to break badly. The same way, the
>>>> fastlock stuff does not track the owner properly in the synchro object.
>>>> We should fix those issues before going further, they may be related to
>>>> the bug described.
>>>>
>>>> Totally, genuinely, 100% untested.
>>>>
>>>> diff --git a/ksrc/nucleus/synch.c b/ksrc/nucleus/synch.c
>>>> index 3a53527..0785533 100644
>>>> --- a/ksrc/nucleus/synch.c
>>>> +++ b/ksrc/nucleus/synch.c
>>>> @@ -424,6 +424,7 @@ xnflags_t xnsynch_acquire(struct xnsynch *synch, 
>>>> xnticks_t timeout,
>>>>                                             XN_NO_HANDLE, threadh);
>>>>  
>>>>            if (likely(fastlock == XN_NO_HANDLE)) {
>>>> +                  xnsynch_set_owner(synch, thread);
>>>>                    xnthread_inc_rescnt(thread);
>>>>                    xnthread_clear_info(thread,
>>>>                                        XNRMID | XNTIMEO | XNBREAK);
>>>> @@ -718,7 +719,7 @@ struct xnthread *xnsynch_release(struct xnsynch *synch)
>>>>  
>>>>    XENO_BUGON(NUCLEUS, !testbits(synch->status, XNSYNCH_OWNER));
>>>>  
>>>> -  lastowner = xnpod_current_thread();
>>>> +  lastowner = synch->owner ?: xnpod_current_thread();
>>>>    xnthread_dec_rescnt(lastowner);
>>>>    XENO_BUGON(NUCLEUS, xnthread_get_rescnt(lastowner) < 0);
>>>>    lastownerh = xnthread_handle(lastowner);
>>>>
>>>
>>> That's maybe another problem, need to check.
>>>
>>> Back to the original issue: with fastlock, kernel space has absolutely
>>> no clue about how many locks user space may hold - unless someone is
>>> contending for all those locks. IOW, you can't reliably track resource
>>> ownership at kernel level without user space help out. The current way
>>> it helps (enforced syscalls of XNOTHER threads) is insufficient.
>>
>> The thing is: we don't care about knowing how many locks some
>> non-current thread owns. What the nucleus wants to know is whether the
>> _current user-space_ thread owns a lock, which is enough for the
>> autorelax management. This restricted scope makes the logic fine.
> 
> Nope, this does not work for threads that undergo policy changes (see
> reply to Gilles).

Is it a really useful use-cache?

-- 
                                                                Gilles.

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