From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4E1F581C.4050809@domain.hid> Date: Thu, 14 Jul 2011 22:57:00 +0200 From: Jan Kiszka MIME-Version: 1.0 References: <4E1B469A.8000703@domain.hid> <4E1B4AC0.80506@domain.hid> <4E1B4C19.2070205@domain.hid> <4E1B542B.2010906@domain.hid> <4E1B5638.1050005@domain.hid> <4E1B56E0.20109@domain.hid> <4E1B57D1.1070401@domain.hid> <4E1B5860.1000309@domain.hid> <4E1B5944.5030408@domain.hid> <4E1BEC9F.1020404@domain.hid> <4E1BF619.6010609@domain.hid> <4E1C2912.9050605@domain.hid> <4E1C2959.8080004@domain.hid> <4E1C2A2D.9090602@domain.hid> <4E1C2AA5.6060208@domain.hid> <4E1C2B44.5060907@domain.hid> <4E1C2B8F.5080700@domain.hid> <4E1C2F56.8020103@domain.hid> <4E1C302A.8050309@domain.hid> <4E1C3301.2030203@domain.hid> <4E1C3672.1030104@domain.hid> <4E1C36EE.70803@domain.hid> <4E1C38CE.7090202@domain.hid> <4E1C3A5D.3020700@domain.hid> <4E1C44B4.50106@domain.hid> <4E1C8508.5010400@domain.hid> <4E1C858A.7070403@domain.hid> <4E1C86A1.6030707@domain.hid> <4E1C87BB.7000307@domain.hid> <4E1DE646.1090900@domain.hid> <4E1DEC58.4000901@domain.hid> <4E1DEE27.7030900@domain.hid> In-Reply-To: <4E1DEE27.7030900@domain.hid> Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="------------enig3A764201DC7E14714C167E97" Sender: jan.kiszka@domain.hid Subject: Re: [Xenomai-core] [Xenomai-git] Jan Kiszka : nucleus: Fix race between gatekeeper and thread deletion List-Id: Xenomai life and development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Gilles Chanteperdrix Cc: Xenomai core This is an OpenPGP/MIME signed message (RFC 2440 and 3156) --------------enig3A764201DC7E14714C167E97 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On 2011-07-13 21:12, Gilles Chanteperdrix wrote: > On 07/13/2011 09:04 PM, Jan Kiszka wrote: >> On 2011-07-13 20:39, Gilles Chanteperdrix wrote: >>> On 07/12/2011 07:43 PM, Jan Kiszka wrote: >>>> On 2011-07-12 19:38, Gilles Chanteperdrix wrote: >>>>> On 07/12/2011 07:34 PM, Jan Kiszka wrote: >>>>>> On 2011-07-12 19:31, Gilles Chanteperdrix wrote: >>>>>>> On 07/12/2011 02:57 PM, Jan Kiszka wrote: >>>>>>>> xnlock_put_irqrestore(&nklock, s); >>>>>>>> xnpod_schedule(); >>>>>>>> } >>>>>>>> @@ -1036,6 +1043,7 @@ redo: >>>>>>>> * to process this signal anyway. >>>>>>>> */ >>>>>>>> if (rthal_current_domain =3D=3D rthal_root_domain) { >>>>>>>> + XENO_BUGON(NUCLEUS, xnthread_test_info(thread, XNATOMIC)); >>>>>>> >>>>>>> Misleading dead code again, XNATOMIC is cleared not ten lines abo= ve. >>>>>> >>>>>> Nope, I forgot to remove that line. >>>>>> >>>>>>> >>>>>>>> if (XENO_DEBUG(NUCLEUS) && (!signal_pending(this_task) >>>>>>>> || this_task->state !=3D TASK_RUNNING)) >>>>>>>> xnpod_fatal >>>>>>>> @@ -1044,6 +1052,8 @@ redo: >>>>>>>> return -ERESTARTSYS; >>>>>>>> } >>>>>>>> =20 >>>>>>>> + xnthread_clear_info(thread, XNATOMIC); >>>>>>> >>>>>>> Why this? I find the xnthread_clear_info(XNATOMIC) right at the r= ight >>>>>>> place at the point it currently is. >>>>>> >>>>>> Nope. Now we either clear XNATOMIC after successful migration or w= hen >>>>>> the signal is about to be sent (ie. in the hook). That way we can = test >>>>>> more reliably (TM) in the gatekeeper if the thread can be migrated= =2E >>>>> >>>>> Ok for adding the XNATOMIC test, because it improves the robustness= , but >>>>> why changing the way XNATOMIC is set and clear? Chances of breaking= >>>>> thing while changing code in this area are really high... >>>> >>>> The current code is (most probably) broken as it does not properly >>>> synchronizes the gatekeeper against a signaled and "runaway" target >>>> Linux task. >>>> >>>> We need an indication if a Linux signal will (or already has) woken = up >>>> the to-be-migrated task. That task may have continued over its conte= xt, >>>> potentially on a different CPU. Providing this indication is the pur= pose >>>> of changing where XNATOMIC is cleared. >>> >>> What about synchronizing with the gatekeeper with a semaphore, as don= e >>> in the first patch you sent, but doing it in xnshadow_harden, as soon= as >>> we detect that we are not back from schedule in primary mode? It seem= s >>> it would avoid any further issue, as we would then be guaranteed that= >>> the thread could not switch to TASK_INTERRUPTIBLE again before the >>> gatekeeper is finished. >> >> The problem is that the gatekeeper tests the task state without holdin= g >> the task's rq lock (which is not available to us without a kernel >> patch). That cannot work reliably as long as we accept signals. That's= >> why I'm trying to move state change and test under nklock. >> >>> >>> What worries me is the comment in xnshadow_harden: >>> >>> * gatekeeper sent us to primary mode. Since >>> * TASK_UNINTERRUPTIBLE is unavailable to us without wrecking >>> * the runqueue's count of uniniterruptible tasks, we just >>> * notice the issue and gracefully fail; the caller will have >>> * to process this signal anyway. >>> */ >>> >>> Does this mean that we can not switch to TASK_UNINTERRUPTIBLE at this= >>> point? Or simply that TASK_UNINTERRUPTIBLE is not available for the >>> business of xnshadow_harden? >>> >> >> TASK_UNINTERRUPTIBLE is not available without patching the kernel's >> scheduler for the reason mentioned in the comment (the scheduler becom= es >> confused and may pick the wrong tasks, IIRC). >=20 > Does not using down/up in the taskexit event handler risk to cause the > same issue? Yes, and that means the first patch is incomplete without something like the second. >=20 >> >> But I would refrain from trying to "improve" the gatekeeper design. I'= ve >> recently mentioned this to Philippe offlist: For Xenomai 3 with some >> ipipe v3, we must rather patch schedule() to enable zero-switch domain= >> migration. Means: enter the scheduler, let it suspend current and pick= >> another task, but then simply escalate to the RT domain before doing a= ny >> context switch. That's much cheaper than the current design and >> hopefully also less error-prone. >=20 > So, do you want me to merge your for-upstream branch? You may merge up to for-upstream^, ie. without any gatekeeper fixes. I strongly suspect that there are still more races in the migration path. The crashes we face even with all patches applied may be related to a shadow task being executed under Linux and Xenomai at the same time.= Jan --------------enig3A764201DC7E14714C167E97 Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.16 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAk4fWB8ACgkQitSsb3rl5xQDRwCg4NRowxUTilDNt3FGnicFeK6n +ccAoMEmK6LsyfUPYrD3eRyfviHilyKd =ic14 -----END PGP SIGNATURE----- --------------enig3A764201DC7E14714C167E97--