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,
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.


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