Re: [Xenomai-core] Fragile lock usage tracking for auto-relax

[Jan Kiszka <jan.kiszka@domain.hid>]

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.

Alternatively to plain counting of ownership in user space, we could
adopt mainline's robust mutex mechanism (a user space maintained list)
that solves the release-all-ownerships issue. But I haven't looked into
details yet.

Jan

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