From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4E1C2B8F.5080700@domain.hid> Date: Tue, 12 Jul 2011 13:10:07 +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> In-Reply-To: <4E1C2B44.5060907@domain.hid> Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="------------enigBBAC49324A3B47AC8C170DCD" 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) --------------enigBBAC49324A3B47AC8C170DCD Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On 2011-07-12 13:08, Gilles Chanteperdrix wrote: > On 07/12/2011 01:06 PM, Jan Kiszka wrote: >> On 2011-07-12 13:04, Gilles Chanteperdrix wrote: >>> On 07/12/2011 01:00 PM, Jan Kiszka wrote: >>>> On 2011-07-12 12:59, Gilles Chanteperdrix wrote: >>>>> On 07/12/2011 09:22 AM, Jan Kiszka wrote: >>>>>> On 2011-07-12 08:41, Gilles Chanteperdrix wrote: >>>>>>> On 07/11/2011 10:12 PM, Jan Kiszka wrote: >>>>>>>> On 2011-07-11 22:09, Gilles Chanteperdrix wrote: >>>>>>>>> On 07/11/2011 10:06 PM, Jan Kiszka wrote: >>>>>>>>>> On 2011-07-11 22:02, Gilles Chanteperdrix wrote: >>>>>>>>>>> On 07/11/2011 09:59 PM, Jan Kiszka wrote: >>>>>>>>>>>> On 2011-07-11 21:51, Gilles Chanteperdrix wrote: >>>>>>>>>>>>> On 07/11/2011 09:16 PM, Jan Kiszka wrote: >>>>>>>>>>>>>> On 2011-07-11 21:10, Jan Kiszka wrote: >>>>>>>>>>>>>>> On 2011-07-11 20:53, Gilles Chanteperdrix wrote: >>>>>>>>>>>>>>>> On 07/08/2011 06:29 PM, GIT version control wrote: >>>>>>>>>>>>>>>>> @@ -2528,6 +2534,22 @@ static inline void do_taskexit_e= vent(struct task_struct *p) >>>>>>>>>>>>>>>>> magic =3D xnthread_get_magic(thread); >>>>>>>>>>>>>>>>> =20 >>>>>>>>>>>>>>>>> xnlock_get_irqsave(&nklock, s); >>>>>>>>>>>>>>>>> + >>>>>>>>>>>>>>>>> + gksched =3D thread->gksched; >>>>>>>>>>>>>>>>> + if (gksched) { >>>>>>>>>>>>>>>>> + xnlock_put_irqrestore(&nklock, s); >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> Are we sure irqs are on here? Are you sure that what is = needed is not an >>>>>>>>>>>>>>>> xnlock_clear_irqon? >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> We are in the context of do_exit. Not only IRQs are on, a= lso preemption. >>>>>>>>>>>>>>> And surely no nklock is held. >>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> Furthermore, I do not understand how we >>>>>>>>>>>>>>>> "synchronize" with the gatekeeper, how is the gatekeeper= garanteed to >>>>>>>>>>>>>>>> wait for this assignment? >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> The gatekeeper holds the gksync token while it's active. = We request it, >>>>>>>>>>>>>>> thus we wait for the gatekeeper to become idle again. Whi= le it is idle, >>>>>>>>>>>>>>> we reset the queued reference - but I just realized that = this may tramp >>>>>>>>>>>>>>> on other tasks' values. I need to add a check that the va= lue to be >>>>>>>>>>>>>>> null'ified is actually still ours. >>>>>>>>>>>>>> >>>>>>>>>>>>>> Thinking again, that's actually not a problem: gktarget is= only needed >>>>>>>>>>>>>> while gksync is zero - but then we won't get hold of it an= yway and, >>>>>>>>>>>>>> thus, can't cause any damage. >>>>>>>>>>>>> >>>>>>>>>>>>> Well, you make it look like it does not work. From what I u= nderstand, >>>>>>>>>>>>> what you want is to set gktarget to null if a task being ha= rdened is >>>>>>>>>>>>> destroyed. But by waiting for the semaphore, you actually w= ait for the >>>>>>>>>>>>> harden to be complete, so setting to NULL is useless. Or am= I missing >>>>>>>>>>>>> something else? >>>>>>>>>>>> >>>>>>>>>>>> Setting to NULL is probably unneeded but still better than r= ely on the >>>>>>>>>>>> gatekeeper never waking up spuriously and then dereferencing= a stale >>>>>>>>>>>> pointer. >>>>>>>>>>>> >>>>>>>>>>>> The key element of this fix is waitng on gksync, thus on the= completion >>>>>>>>>>>> of the non-RT part of the hardening. Actually, this part usu= ally fails >>>>>>>>>>>> as the target task received a termination signal at this poi= nt. >>>>>>>>>>> >>>>>>>>>>> Yes, but since you wait on the completion of the hardening, t= he test >>>>>>>>>>> if (target &&...) in the gatekeeper code will always be true,= because at >>>>>>>>>>> this point the cleanup code will still be waiting for the sem= aphore. >>>>>>>>>> >>>>>>>>>> Yes, except we will ever wake up the gatekeeper later on witho= ut an >>>>>>>>>> updated gktarget, ie. spuriously. Better safe than sorry, this= is hairy >>>>>>>>>> code anyway (hopefully obsolete one day). >>>>>>>>> >>>>>>>>> The gatekeeper is not woken up by posting the semaphore, the ga= tekeeper >>>>>>>>> is woken up by the thread which is going to be hardened (and th= is thread >>>>>>>>> is the one which waits for the semaphore). >>>>>>>> >>>>>>>> All true. And what is the point? >>>>>>> >>>>>>> The point being, would not something like this patch be sufficien= t? >>>>>>> >>>>>>> diff --git a/ksrc/nucleus/shadow.c b/ksrc/nucleus/shadow.c >>>>>>> index 01f4200..4742c02 100644 >>>>>>> --- a/ksrc/nucleus/shadow.c >>>>>>> +++ b/ksrc/nucleus/shadow.c >>>>>>> @@ -2527,6 +2527,18 @@ static inline void do_taskexit_event(struc= t >>>>>>> task_struct *p) >>>>>>> magic =3D xnthread_get_magic(thread); >>>>>>> >>>>>>> xnlock_get_irqsave(&nklock, s); >>>>>>> + if (xnthread_test_info(thread, XNATOMIC)) { >>>>>>> + struct xnsched *gksched =3D xnpod_sched_slot(task_cpu(p)); >>>>>> >>>>>> That's not reliable, the task might have been migrated by Linux in= the >>>>>> meantime. We must use the stored gksched. >>>>>> >>>>>>> + xnlock_put_irqrestore(&nklock, s); >>>>>>> + >>>>>>> + /* Thread is in flight to primary mode, wait for the >>>>>>> + gatekeeper to be done with it. */ >>>>>>> + down(&gksched->gksync); >>>>>>> + up(&gksched->gksync); >>>>>>> + >>>>>>> + xnlock_get_irqsave(&nklock, s); >>>>>>> + } >>>>>>> + >>>>>>> /* Prevent wakeup call from xnshadow_unmap(). */ >>>>>>> xnshadow_thrptd(p) =3D NULL; >>>>>>> xnthread_archtcb(thread)->user_task =3D NULL; >>>>>>> >>>>>> >>>>>> Again, setting gktarget to NULL and testing for NULL is simply saf= er, >>>>>> and I see no gain in skipping that. But if you prefer the >>>>>> micro-optimization, I'll drop it. >>>>> >>>>> Could not we use an info bit instead of adding a pointer? >>>>> >>>> >>>> "That's not reliable, the task might have been migrated by Linux in = the >>>> meantime. We must use the stored gksched." >>> >>> I mean add another info bit to mean that the task is queued for wakeu= p >>> by the gatekeeper. >>> >>> XNGKQ, or something. >> >> What additional value does it provide to gksched !=3D NULL? We need th= at >> pointer anyway to identify the gatekeeper that holds a reference. >=20 > No, the scheduler which holds the reference is xnpod_sched_slot(task_cp= u(p)) And that is still not reliable, the task may have been pushed to a different CPU in the meantime. Jan --------------enigBBAC49324A3B47AC8C170DCD 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/ iEYEARECAAYFAk4cK48ACgkQitSsb3rl5xSWmwCeLNbv4dJDLUiG3iqEG+TtiCdk VnMAmwcs4B/9QO3AMq4499Uk8ZQf98Y/ =XUBd -----END PGP SIGNATURE----- --------------enigBBAC49324A3B47AC8C170DCD--