From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <53346E91.8080903@web.de> Date: Thu, 27 Mar 2014 19:31:45 +0100 From: Jan Kiszka MIME-Version: 1.0 References: <53316C4C.6000708@sigmatek.at> <53335D9A.9040902@xenomai.org> <5333F22F.70805@xenomai.org> <53345143.7000009@xenomai.org> <53346835.4030202@web.de> <53346CF7.2020805@xenomai.org> In-Reply-To: <53346CF7.2020805@xenomai.org> Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Subject: Re: [Xenomai] Rescnt imbalance in rtdm_mutex_timedlock List-Id: Discussions about the Xenomai project List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Philippe Gerum , Henri Roosen Cc: "Xenomai@xenomai.org" On 2014-03-27 19:24, Philippe Gerum wrote: > On 03/27/2014 07:04 PM, Jan Kiszka wrote: >> On 2014-03-27 17:26, Philippe Gerum wrote: >>> On 03/27/2014 04:02 PM, Henri Roosen wrote: >>>> >>>> >>>> >>>> On Thu, Mar 27, 2014 at 10:41 AM, Philippe Gerum >>> > wrote: >>>> >>>> On 03/27/2014 12:07 AM, Gilles Chanteperdrix wrote: >>>> >>>> On 03/26/2014 05:49 PM, Henri Roosen wrote: >>>> >>>> On Tue, Mar 25, 2014 at 12:45 PM, Erwin Pranz >>>> >>> >__wrote: >>>> >>>> Hello, >>>> >>>> I am using rtdm_mutex_lock/rtdm_mutex___unlock in a >>>> rtdm >>>> driver and I get a >>>> signal SIGDEBUG_MIGRATE_PRIOINV when I call the driver >>>> from a priority-0 >>>> thread.The signal is sent in xnsynch_release_thread >>>> when >>>> rescnt is 0 on a >>>> XNOTHER thread, which means that rescnt is not >>>> balanced. >>>> >>>> I think the reason why rescnt is not balanced is in >>>> rtdm_mutex_timedlock: >>>> when the owner of the mutex is NULL, the owner is >>>> changed to the calling >>>> thread, but rescnt is not increased. >>>> >>>> I suggest the following fix in rtdm_mutex_timedlock: >>>> - else if >>>> (likely(xnsynch_owner(&mutex->__synch_base) =3D=3D NU= LL)) >>>> + else if >>>> (likely(xnsynch_owner(&mutex->__synch_base) =3D=3D >>>> NULL)) { >>>> >>>> xnsynch_set_owner(&mutex->__synch_base, >>>> curr_thread); >>>> + if (xnthread_test_state(curr___thread, >>>> XNOTHER)) >>>> + = >>>> xnthread_inc_rescnt(curr___thread); >>>> + } >>>> >>>> >>>> Hi Gilles, >>>> >>>> Could you maybe elaborate on this? It seems the resource >>>> counting is not in >>>> balance. It would be great to know if this the correct way >>>> to fix it. >>>> >>>> The problem should be easily reproducible by having >>>> just one >>>> shadowed >>>> priority 0 thread that somehow calls rtdm_mutex_lock >>>> directly followed by >>>> rtdm_mutex_unlock. This will trigger the code in >>>> ksrc/nucleus/synch.c that >>>> was added by commit-486afd15, which sends >>>> SIGDEBUG_MIGRATE_PRIOINV on >>>> Xenomai v2.6.2.1. It shouldn't send any signal in that >>>> case, >>>> as nothing is >>>> wrong. >>>> >>>> Note we are on Xenomai v2.6.2.1. Xenomai v2.6.3 sends (the >>>> better name for >>>> it) SIGDEBUG_RESCNT_IMBALANCE. >>>> >>>> >>>> To be completely frank, I expected Jan to answer this mail >>>> as this >>>> concerns RTDM. Anyway, I had a look into it, and: >>>> - the RTDM skin is not alone in this case, several skins >>>> implement the >>>> same pattern (if owner =3D=3D NULL, set owner, then return); >>>> - adding calls to xnthread_inc_rescnt everywhere does not seem >>>> right, I >>>> would prefer to move this inside some xnsynch_ service. The >>>> best >>>> candidate I found for that is xnsynch_set_owner, though I >>>> am not >>>> sure if >>>> this will work, especially with fastsynchs. >>>> >>>> >>>> >>>> Indeed this wouldn't. I suspect the skins mentioned did not have >>>> access to xnsynch_acquire(), and were not updated when the >>>> auto-relax feature was introduced for XNOTHER threads. >>>> >>>> We could provide a helper for fast grabbing a mutex which has no >>>> fast locking support shared with userland, saving a call to >>>> xnsynch_acquire() in the non-contended case, e.g. >>>> >>>> diff --git a/include/nucleus/synch.h b/include/nucleus/synch.h >>>> index 7deae21..87456ea 100644 >>>> --- a/include/nucleus/synch.h >>>> +++ b/include/nucleus/synch.h >>>> @@ -192,6 +192,22 @@ static inline void xnsynch_set_owner(struct >>>> xnsynch *synch, >>>> synch->owner =3D thread; >>>> } >>>> >>>> +static inline int xnsynch_fast_grab(struct xnsynch *synch, >>>> + struct xnthread *thread) >>>> +{ >>>> + XENO_BUGON(NUCLEUS, xnsynch_fastlock_p(synch)); >>>> + >>>> + if (likely(xnsynch_owner(synch) !=3D NULL)) >>>> + return 0; >>>> + >>>> + xnsynch_set_owner(synch, thread); >>>> + >>>> + if (xnthread_test_state(thread, XNOTHER)) >>>> + xnthread_inc_rescnt(thread); >>>> + >>>> + return 1; >>>> +} >>>> + >>>> static inline void xnsynch_register_cleanup(__struct xnsynch >>>> *synch, >>>> void >>>> (*handler)(struct >>>> xnsynch *)) >>>> { >>>> diff --git a/ksrc/skins/rtdm/drvlib.c b/ksrc/skins/rtdm/drvlib.c >>>> index 391d45f..338005d 100644 >>>> --- a/ksrc/skins/rtdm/drvlib.c >>>> +++ b/ksrc/skins/rtdm/drvlib.c >>>> @@ -1538,9 +1538,7 @@ int rtdm_mutex_timedlock(rtdm___mutex_t >>>> *mutex, nanosecs_rel_t timeout, >>>> if (unlikely(xnsynch_test_flags(&__mutex->synch_base, >>>> RTDM_SYNCH_DELETED))) >>>> err =3D -EIDRM; >>>> >>>> - else if (likely(xnsynch_owner(&mutex->__synch_base) =3D= =3D >>>> NULL)) >>>> - xnsynch_set_owner(&mutex->__synch_base, >>>> curr_thread); >>>> - else { >>>> + else if (!xnsynch_fast_grab(&mutex->__synch_base, >>>> curr_thread)) { >>>> /* Redefinition to clarify XENO_ASSERT output */ >>>> #define mutex_owner >>>> xnsynch_owner(&mutex->synch___base) >>>> XENO_ASSERT(RTDM, mutex_owner !=3D curr_thread, >>>> >>>> The VxWorks and VRTX skins would have to use it as well. >>>> >>>> >>>> Thanks for the replies! We'll give this patch a try for the RTDM case. >>>> >>> >>> Minor points, but a compiler hint above was inverted and eventually >>> useless, and the naming not descriptive enough. Take #2: >>> >>> diff --git a/include/nucleus/synch.h b/include/nucleus/synch.h >>> index 7deae21..09ebdbf 100644 >>> --- a/include/nucleus/synch.h >>> +++ b/include/nucleus/synch.h >>> @@ -192,6 +192,22 @@ static inline void xnsynch_set_owner(struct >>> xnsynch *synch, >>> synch->owner =3D thread; >>> } >>> >>> +static inline int xnsynch_try_grab(struct xnsynch *synch, >>> + struct xnthread *thread) >>> +{ >>> + XENO_BUGON(NUCLEUS, xnsynch_fastlock_p(synch)); >>> + >>> + if (xnsynch_owner(synch) !=3D NULL) >> >> if (unlikely(...)) - locks are generally unlikely to be contended >> (unless something is suboptimally designed at the user site). >> > = > I'm unsure we really need to take a chance at giving such hint. Modern > CPUs tend to have branch prediction right. Then likely/unlikely will be emptied by Linux. But I'm not aware of any plans in the kernel community to phase out this tag. Jan -------------- next part -------------- A non-text attachment was scrubbed... Name: signature.asc Type: application/pgp-signature Size: 263 bytes Desc: OpenPGP digital signature URL: