Re: [PATCH v4 2/5] x86, traps: Track entry into and exit from IST context

From: Andy Lutomirski
Date: Fri Nov 21 2014 - 17:19:45 EST

On Fri, Nov 21, 2014 at 2:07 PM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> On Fri, Nov 21, 2014 at 01:32:50PM -0800, Andy Lutomirski wrote:
>> On Fri, Nov 21, 2014 at 1:26 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> > We currently pretend that IST context is like standard exception
>> > context, but this is incorrect. IST entries from userspace are like
>> > standard exceptions except that they use per-cpu stacks, so they are
>> > atomic. IST entries from kernel space are like NMIs from RCU's
>> > perspective -- they are not quiescent states even if they
>> > interrupted the kernel during a quiescent state.
>> >
>> > Add and use ist_enter and ist_exit to track IST context. Even
>> > though x86_32 has no IST stacks, we track these interrupts the same
>> > way.
>> I should add:
>> I have no idea why RCU read-side critical sections are safe inside
>> __do_page_fault today. It's guarded by exception_enter(), but that
>> doesn't do anything if context tracking is off, and context tracking
>> is usually off. What am I missing here?
> Ah! There are three cases:
> 1. Context tracking is off on a non-idle CPU. In this case, RCU is
> still paying attention to CPUs running in both userspace and in
> the kernel. So if a page fault happens, RCU will be set up to
> notice any RCU read-side critical sections.
> 2. Context tracking is on on a non-idle CPU. In this case, RCU
> might well be ignoring userspace execution: NO_HZ_FULL and
> all that. However, as you pointed out, in this case the
> context-tracking code lets RCU know that we have entered the
> kernel, which means that RCU will again be paying attention to
> RCU read-side critical sections.
> 3. The CPU is idle. In this case, RCU is ignoring the CPU, so
> if we take a page fault when context tracking is off, life
> will be hard. But the kernel is not supposed to take page
> faults in the idle loop, so this is not a problem.

I guess so, as long as there are really no page faults in the idle loop.

There are, however, machine checks in the idle loop, and maybe kprobes
(haven't checked), so I think this patch might fix real bugs.

> Just out of curiosity... Can an NMI occur in IST context? If it can,
> I need to make rcu_nmi_enter() and rcu_nmi_exit() deal properly with
> nested calls.

Yes, and vice versa. That code looked like it handled nesting
correctly, but I wasn't entirely sure.

Also, just to make sure: are we okay if rcu_nmi_enter() is called
before exception_enter if context tracking is on and we came directly
from userspace?

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