On 05/31/2011 06:29 PM, 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.
It looks to me like xnsynch_fast_release uses cmpxchg, so, will not set
the owner to NULL if the current owner is not the thread releasing the
mutex. Is it not sufficient?
--
Gilles.
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