From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4E1C2AA5.6060208@domain.hid> Date: Tue, 12 Jul 2011 13:06:13 +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> In-Reply-To: <4E1C2A2D.9090602@domain.hid> Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="------------enig28AC0316A2313566A2AD4558" 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) --------------enig28AC0316A2313566A2AD4558 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable 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_eve= nt(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 ne= eded is not an >>>>>>>>>>>>>> xnlock_clear_irqon? >>>>>>>>>>>>> >>>>>>>>>>>>> We are in the context of do_exit. Not only IRQs are on, als= o preemption. >>>>>>>>>>>>> And surely no nklock is held. >>>>>>>>>>>>> >>>>>>>>>>>>>> Furthermore, I do not understand how we >>>>>>>>>>>>>> "synchronize" with the gatekeeper, how is the gatekeeper g= aranteed 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. While= it is idle, >>>>>>>>>>>>> we reset the queued reference - but I just realized that th= is may tramp >>>>>>>>>>>>> on other tasks' values. I need to add a check that the valu= e to be >>>>>>>>>>>>> null'ified is actually still ours. >>>>>>>>>>>> >>>>>>>>>>>> Thinking again, that's actually not a problem: gktarget is o= nly needed >>>>>>>>>>>> while gksync is zero - but then we won't get hold of it anyw= ay and, >>>>>>>>>>>> thus, can't cause any damage. >>>>>>>>>>> >>>>>>>>>>> Well, you make it look like it does not work. From what I und= erstand, >>>>>>>>>>> what you want is to set gktarget to null if a task being hard= ened is >>>>>>>>>>> destroyed. But by waiting for the semaphore, you actually wai= t 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 rel= y 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 c= ompletion >>>>>>>>>> of the non-RT part of the hardening. Actually, this part usual= ly fails >>>>>>>>>> as the target task received a termination signal at this point= =2E >>>>>>>>> >>>>>>>>> Yes, but since you wait on the completion of the hardening, the= test >>>>>>>>> if (target &&...) in the gatekeeper code will always be true, b= ecause at >>>>>>>>> this point the cleanup code will still be waiting for the semap= hore. >>>>>>>> >>>>>>>> Yes, except we will ever wake up the gatekeeper later on without= an >>>>>>>> updated gktarget, ie. spuriously. Better safe than sorry, this i= s hairy >>>>>>>> code anyway (hopefully obsolete one day). >>>>>>> >>>>>>> The gatekeeper is not woken up by posting the semaphore, the gate= keeper >>>>>>> is woken up by the thread which is going to be hardened (and this= 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 sufficient?= >>>>> >>>>> 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(struct >>>>> 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 t= he >>>> 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 safer= , >>>> 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 th= e >> meantime. We must use the stored gksched." >=20 > I mean add another info bit to mean that the task is queued for wakeup > by the gatekeeper. >=20 > XNGKQ, or something. What additional value does it provide to gksched !=3D NULL? We need that pointer anyway to identify the gatekeeper that holds a reference. Jan --------------enig28AC0316A2313566A2AD4558 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/ iEYEARECAAYFAk4cKqUACgkQitSsb3rl5xTjygCgjuj4xsLOe94UDoColgEAx5a1 VP4AoLLiLVaOFhUoq3ZfW+vHkeNNBYUT =iJGa -----END PGP SIGNATURE----- --------------enig28AC0316A2313566A2AD4558--