From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4A4BA334.8090701@domain.hid> Date: Wed, 01 Jul 2009 19:56:04 +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> In-Reply-To: <4A4B8912.1060700@domain.hid> Content-Type: text/plain; charset=UTF-8 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: Gilles Chanteperdrix Cc: xenomai-core 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); } Jan -- Siemens AG, Corporate Technology, CT SE 2 Corporate Competence Center Embedded Linux