From mboxrd@z Thu Jan 1 00:00:00 1970 From: Philippe Gerum In-Reply-To: <1246472157.7803.62.camel@domain.hid> 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> Content-Type: text/plain Date: Wed, 01 Jul 2009 20:27:13 +0200 Message-Id: <1246472833.7803.67.camel@domain.hid> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit 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: Jan Kiszka Cc: xenomai-core On Wed, 2009-07-01 at 20:15 +0200, 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 digging, 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 kernel > > >>>>>>> tries to fixup the wrong mm. And that means we have some open race > > >>>>>>> window between updating cr3 and active_mm somewhere (isn't switch_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 we 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 context > > >>>>>> 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 more > > >>>> 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 found, > > >> We have two direct callers of switch_mm in sched.c and one in fs/aio.c. > > >> Both need protection (I pushed IRQ disabling into switch_mm), but that > > >> 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 between > > >>> 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 what I > > >>> understand, switch_mm called by Xenomai upon return to root would re-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 is > > >>> 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 > > > != 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 = current; > > + unsigned long flags; > > > > task_lock(tsk); > > active_mm = tsk->active_mm; > > atomic_inc(&mm->mm_count); > > + local_irq_save_hw_cond(flags); > > tsk->mm = mm; > > tsk->active_mm = mm; > > switch_mm(active_mm, mm, tsk); > > + local_irq_restore_hw_cond(flags); > > task_unlock(tsk); > > > > 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_struct *prev, > > next->active_mm = 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); > > + } > > > > if (unlikely(!prev->mm)) { > > prev->active_mm = NULL; > > @@ -6406,8 +6410,12 @@ void idle_task_exit(void) > > > > BUG_ON(cpu_online(smp_processor_id())); > > > > - if (mm != &init_mm) > > + if (mm != &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); > > } > > 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. > Btw, this how the powerpc port works in locked switch mode, and this particular bug does not apply in unlocked switch mode anyway; this is why we don't have that problem on this arch. The ARM port always works in unlocked switch mode IIRC for latency reasons, so this should be a non-issue here as well. Gilles, would you confirm this? > > > > > > Jan > > -- Philippe.