From mboxrd@z Thu Jan 1 00:00:00 1970 From: Philippe Gerum In-Reply-To: <4A4BA334.8090701@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> Content-Type: text/plain Date: Wed, 01 Jul 2009 20:15:57 +0200 Message-Id: <1246472157.7803.62.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 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. > > > Jan > -- Philippe.