From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4E1BF619.6010609@domain.hid> Date: Tue, 12 Jul 2011 09:22:01 +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> In-Reply-To: <4E1BEC9F.1020404@domain.hid> Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="------------enig53AF49953FECC47A1EAA09C8" 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) --------------enig53AF49953FECC47A1EAA09C8 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable 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_event(s= truct 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, also pr= eemption. >>>>>>>>> And surely no nklock is held. >>>>>>>>> >>>>>>>>>> Furthermore, I do not understand how we >>>>>>>>>> "synchronize" with the gatekeeper, how is the gatekeeper garan= teed to >>>>>>>>>> wait for this assignment? >>>>>>>>> >>>>>>>>> The gatekeeper holds the gksync token while it's active. We req= uest 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 this m= ay tramp >>>>>>>>> on other tasks' values. I need to add a check that the value 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 anyway a= nd, >>>>>>>> thus, can't cause any damage. >>>>>>> >>>>>>> Well, you make it look like it does not work. From what I underst= and, >>>>>>> what you want is to set gktarget to null if a task being hardened= is >>>>>>> destroyed. But by waiting for the semaphore, you actually wait fo= r the >>>>>>> harden to be complete, so setting to NULL is useless. Or am I mis= sing >>>>>>> something else? >>>>>> >>>>>> Setting to NULL is probably unneeded but still better than rely on= the >>>>>> gatekeeper never waking up spuriously and then dereferencing a sta= le >>>>>> pointer. >>>>>> >>>>>> The key element of this fix is waitng on gksync, thus on the compl= etion >>>>>> of the non-RT part of the hardening. Actually, this part usually f= ails >>>>>> as the target task received a termination signal at this point. >>>>> >>>>> Yes, but since you wait on the completion of the hardening, the tes= t >>>>> if (target &&...) in the gatekeeper code will always be true, becau= se at >>>>> this point the cleanup code will still be waiting for the semaphore= =2E >>>> >>>> Yes, except we will ever wake up the gatekeeper later on without an >>>> updated gktarget, ie. spuriously. Better safe than sorry, this is ha= iry >>>> code anyway (hopefully obsolete one day). >>> >>> The gatekeeper is not woken up by posting the semaphore, the gatekeep= er >>> is woken up by the thread which is going to be hardened (and this thr= ead >>> is the one which waits for the semaphore). >> >> All true. And what is the point? >=20 > The point being, would not something like this patch be sufficient? >=20 > 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); >=20 > 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; >=20 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. Jan --------------enig53AF49953FECC47A1EAA09C8 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/ iEYEARECAAYFAk4b9hkACgkQitSsb3rl5xTFwgCg7dqfunNB4Cmd7UzeXOiQPPDm 1mgAn1aj5TmXE5OPXjgR2iIUx7GhgY2+ =kE+D -----END PGP SIGNATURE----- --------------enig53AF49953FECC47A1EAA09C8--