From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Subject: Re: [PATCH] powerpc - Initialize the irq radix tree earlier From: Michael Ellerman To: Sebastien Dugue In-Reply-To: <20080731152606.5ae7c379@bull.net> References: <1217497241-10685-1-git-send-email-sebastien.dugue@bull.net> <1217497241-10685-2-git-send-email-sebastien.dugue@bull.net> <1217504456.9817.22.camel@localhost> <20080731140002.31bbe4a0@bull.net> <1217509104.19050.11.camel@localhost> <1217509299.19050.15.camel@localhost> <20080731152606.5ae7c379@bull.net> Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="=-vW9406vhric++rHHzA/k" Date: Thu, 31 Jul 2008 23:39:26 +1000 Message-Id: <1217511566.19050.26.camel@localhost> Mime-Version: 1.0 Cc: tinytim@us.ibm.com, linux-rt-users@vger.kernel.org, jean-pierre.dion@bull.net, rostedt@goodmis.org, linux-kernel@vger.kernel.org, linuxppc-dev@ozlabs.org, paulus@samba.org, gilles.carry@ext.bull.net, tglx@linutronix.de Reply-To: michael@ellerman.id.au List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , --=-vW9406vhric++rHHzA/k Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable On Thu, 2008-07-31 at 15:26 +0200, Sebastien Dugue wrote: > On Thu, 31 Jul 2008 23:01:39 +1000 Michael Ellerman wrote: >=20 > > On Thu, 2008-07-31 at 22:58 +1000, Michael Ellerman wrote: > > > On Thu, 2008-07-31 at 14:00 +0200, Sebastien Dugue wrote: > > > > On Thu, 31 Jul 2008 21:40:56 +1000 Michael Ellerman wrote: > > > > >=20 > > > > > This boot ordering stuff is pretty hairy, so I might have missed > > > > > something, but this is how the code is ordered AFAICT: > > > > > =EF=BB=BF > > > > > start_kernel() > > > > > init_IRQ() > > > > > ... > > > > > local_irq_enable() > > > > > ... > > > > > rest_init() > > > > > kernel_thread() > > > > > kernel_init() > > > > > smp_prepare_cpus() > > > > > smp_xics_probe() (via smp_ops->probe()) > > > > >=20 > > > > >=20 > > > > > What's stopping us from taking an irq between local_irq_enable() = and > > > > > smp_xics_probe() ? Is it just that no one's request_irq()'ed the= m yet? > > > >=20 > > > > It's hairy, I agree, but as you've mentioned no one has done a re= quest_irq() > > > > at that point. The first one to do it is smp_xics_probe() for the I= PI. > > >=20 > > > Hmm, I don't think that's strong enough. I can trivially cause irqs t= o > > > fire during a kexec reboot just by mashing the keyboard. > > >=20 > > > And during a kdump boot all sorts of stuff could be firing. Even duri= ng > > > a clean boot, from firmware, I don't think we can guarantee that > > > nothing's going to fire. > > >=20 > > > .. after a bit of testing .. > > >=20 > > > It seems it actually works (sort of).=20 > > >=20 > > > xics_remap_irq() calls irq_radix_revmap_lookup(), which calls: > > >=20 > > > ptr =3D radix_tree_lookup(&host->revmap_data.tree, hwirq); > > >=20 > > > And because =EF=BB=BFhost->revmap_data.tree was zalloc'ed we trip on = the first > > > check here: > >=20 > > @#$% ctrl-enter =3D=3D send! > >=20 > > Continuing ... > >=20 > > void *radix_tree_lookup(struct radix_tree_root *root, unsigned long ind= ex) > > { > > unsigned int height, shift; > > struct radix_tree_node *node, **slot; > >=20 > > node =3D rcu_dereference(root->rnode); > > if (node =3D=3D NULL) > > return NULL; > >=20 > > Which means =EF=BB=BFirq_radix_revmap_lookup() will return NO_IRQ, whic= h is cool. >=20 > Which is what I intended so that as long as no IRQ is registered we > return NO_IRQ. >=20 > >=20 > >=20 > > So I think it can fly, as long as we're happy that we can't reverse map > > anything until smp_xics_probe() - and I think that's true, as any irq w= e > > take will be invalid. >=20 > That's true as no IRQs are registered before smp_xics_probe() and for a= ny > interrupt we might get before that, irq_radix_revmap_lookup() will return > NO_IRQ. Cool, we agree :)=20 My only worry is that we might be relying on on the particular radix tree implementation a bit too much. Is it documented somewhere that the /very/ first check is for =EF=BB=BFroot->rnode !=3D NULL, and the rest = of the root may be unintialised? And I think it needs a big fat comment in the irq code saying that it's safe because revmap_data is zalloc'ed, and that means the radix lookup will fail (safely). cheers --=20 Michael Ellerman OzLabs, IBM Australia Development Lab wwweb: http://michael.ellerman.id.au phone: +61 2 6212 1183 (tie line 70 21183) We do not inherit the earth from our ancestors, we borrow it from our children. - S.M.A.R.T Person --=-vW9406vhric++rHHzA/k Content-Type: application/pgp-signature; name=signature.asc Content-Description: This is a digitally signed message part -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.6 (GNU/Linux) iD8DBQBIkcCOdSjSd0sB4dIRAuxGAJ0TguBTpS0p4xdDf+dAZKTcHyTkvQCeNc7S 6UpVTe06vYU5VRVjCJNvKYU= =IyZ5 -----END PGP SIGNATURE----- --=-vW9406vhric++rHHzA/k--