From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4A4BB14B.4030202@domain.hid> Date: Wed, 01 Jul 2009 20:56:11 +0200 From: Jan Kiszka MIME-Version: 1.0 References: <4A48FB71.6070506@domain.hid> <4A49CD81.4060706@domain.hid> <4A49CFF0.7070202@domain.hid> <1246353623.7803.21.camel@domain.hid> <4A49D935.3060900@domain.hid> <1246353913.7803.24.camel@domain.hid> <4A49DA4E.2020604@domain.hid> <1246354047.7803.25.camel@domain.hid> <4A49DC0A.5000208@domain.hid> <4A4A391B.8000700@domain.hid> <4A4B4ED4.6020208@domain.hid> <4A4B558D.20307@domain.hid> <4A4B58E9.4050407@domain.hid> <4A4B5985.3070504@domain.hid> <4A4B8617.5000704@domain.hid> <4A4B8851.9070005@domain.hid> <4A4B8912.1060700@domain.hid> <4A4BA334.8090701@domain.hid> <1246472157.7803.62.camel@domain.hid> In-Reply-To: <1246472157.7803.62.camel@domain.hid> Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="------------enigB12BDD16242A3E25E35DA22C" Sender: jan.kiszka@domain.hid Subject: Re: [Xenomai-core] x86: Endless minor faults List-Id: Xenomai life and development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Philippe Gerum Cc: xenomai-core This is an OpenPGP/MIME signed message (RFC 2440 and 3156) --------------enigB12BDD16242A3E25E35DA22C Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Philippe Gerum wrote: > On Wed, 2009-07-01 at 19:56 +0200, Jan Kiszka wrote: >> Jan Kiszka wrote: >>> Jan Kiszka wrote: >>>> Gilles Chanteperdrix wrote: >>>>> Jan Kiszka wrote: >>>>>> Jan Kiszka wrote: >>>>>>> Gilles Chanteperdrix wrote: >>>>>>>> Jan Kiszka wrote: >>>>>>>>> Jan Kiszka wrote: >>>>>>>>>> It's still unclear what goes on precisely, we are still diggin= g, but the >>>>>>>>>> test system that can produce this is highly contended. >>>>>>>>> Short update: Further instrumentation revealed that cr3 differs= from >>>>>>>>> active_mm->pgd while we are looping over that fault, ie. the ke= rnel >>>>>>>>> tries to fixup the wrong mm. And that means we have some open r= ace >>>>>>>>> window between updating cr3 and active_mm somewhere (isn't swit= ch_mm run >>>>>>>>> in a preemptible manner now?). >>>>>>>> Maybe the rsp is wrong and leads you to the wrong active_mm ? >>>>>>>> >>>>>>>>> As a first shot I disabled CONFIG_IPIPE_DELAYED_ATOMICSW, and w= e are now >>>>>>>>> checking if it makes a difference. Digging deeper into the code= in the >>>>>>>>> meanwhile... >>>>>>>> As you have found out in the mean time, we do not use unlocked c= ontext >>>>>>>> switches on x86. >>>>>>>> >>>>>>> Yes. >>>>>>> >>>>>>> The last question I asked myself (but couldn't answer yet due to = other >>>>>>> activity) was: Where are the local_irq_disable/enable_hw around >>>>>>> switch_mm for its Linux callers? >>>>>> Ha, that's the point: only activate_mm is protected, but we have m= ore >>>>>> spots in 2.6.29 and maybe other kernels, too! >>>>> Ok, I do not see where switch_mm is called with IRQs off. What I fo= und, >>>> We have two direct callers of switch_mm in sched.c and one in fs/aio= =2Ec. >>>> Both need protection (I pushed IRQ disabling into switch_mm), but th= at >>>> is not enough according to current tests. It seems to reduce to >>>> probability of corruption, though. >>>> >>>>> however, is that leave_mm sets the cr3 and just clears >>>>> active_mm->cpu_vm_mask. So, at this point, we have a discrepancy be= tween >>>>> cr3 and active_mm. I do not know what could happen if Xenomai could= >>>>> interrupt leave_mm between the cpu_clear and the write_cr3. From wh= at I >>>>> understand, switch_mm called by Xenomai upon return to root would r= e-set >>>>> the bit, and re-set cr3, which would be set to the kernel cr3 right= >>>>> after that, but this would result in the active_mm.cpu_vm_mask bit = being >>>>> set instead of cleared as expected. So, maybe an irqs off section i= s >>>>> missing in leave_mm. >>>> leave_mm is already protected by its caller smp_invalidate_interrupt= - >>>> but now I'm parsing context_switch /wrt to lazy tlb. >>>> >>> Hmm... lazy tlb: This means a new task is switched in and has active_= mm >>> !=3D mm. But do_page_fault reads task->mm... Just thoughts, no clear >>> picture yet. >>> >> Looking closer at the call sites of switch_mm, I think our the problem= >> is mostly related to use_mm from fs/aio.c (customer is using aio >> heavily). But other callers need protection, too. We are going to test= >> this patch tomorrow: >> >> diff --git a/fs/aio.c b/fs/aio.c >> index 76da125..d90fca3 100644 >> --- a/fs/aio.c >> +++ b/fs/aio.c >> @@ -618,13 +618,16 @@ static void use_mm(struct mm_struct *mm) >> { >> struct mm_struct *active_mm; >> struct task_struct *tsk =3D current; >> + unsigned long flags; >> =20 >> task_lock(tsk); >> active_mm =3D tsk->active_mm; >> atomic_inc(&mm->mm_count); >> + local_irq_save_hw_cond(flags); >> tsk->mm =3D mm; >> tsk->active_mm =3D mm; >> switch_mm(active_mm, mm, tsk); >> + local_irq_restore_hw_cond(flags); >> task_unlock(tsk); >> =20 >> mmdrop(active_mm); >> diff --git a/kernel/sched.c b/kernel/sched.c >> index aa8f86c..8c545a4 100644 >> --- a/kernel/sched.c >> +++ b/kernel/sched.c >> @@ -2668,8 +2668,12 @@ context_switch(struct rq *rq, struct task_struc= t *prev, >> next->active_mm =3D oldmm; >> atomic_inc(&oldmm->mm_count); >> enter_lazy_tlb(oldmm, next); >> - } else >> + } else { >> + unsigned long flags; >> + local_irq_save_hw_cond(flags); >> switch_mm(oldmm, mm, next); >> + local_irq_restore_hw_cond(flags); >> + } >> =20 >> if (unlikely(!prev->mm)) { >> prev->active_mm =3D NULL; >> @@ -6406,8 +6410,12 @@ void idle_task_exit(void) >> =20 >> BUG_ON(cpu_online(smp_processor_id())); >> =20 >> - if (mm !=3D &init_mm) >> + if (mm !=3D &init_mm) { >> + unsigned long flags; >> + local_irq_save_hw_cond(flags); >> switch_mm(mm, &init_mm, current); >> + local_irq_restore_hw_cond(flags); >> + } >> mmdrop(mm); >> } >=20 > Please fix the callee instead of ironing the call sites. This would > avoid further issues as upstream emits additional switch_mm calls over > time, and make ironing activate_mm useless. This was my first idea, too, but it didn't work, see use_mm(). I don't think we will get around reviewing switch_mm users on kernel updates. Jan --------------enigB12BDD16242A3E25E35DA22C Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.9 (GNU/Linux) Comment: Using GnuPG with SUSE - http://enigmail.mozdev.org iEYEARECAAYFAkpLsUsACgkQniDOoMHTA+m6LgCfeWpko1Gk1C25w8W8pWWAUAUD +pkAmwROEahd3X+P3MjRHP0JihjtqgSX =vHf0 -----END PGP SIGNATURE----- --------------enigB12BDD16242A3E25E35DA22C--