Re: [tip:x86/asm] x86/irq, context_tracking: Document how IRQ context tracking works and add an RCU assertion

[Andy Lutomirski <luto@amacapital.net>]

On Tue, Jul 14, 2015 at 4:26 PM, Frederic Weisbecker <fweisbec@gmail.com> wrote:
> On Tue, Jul 07, 2015 at 03:54:32AM -0700, tip-bot for Andy Lutomirski wrote:
>> Commit-ID:  0333a209cbf600e980fc55c24878a56f25f48b65
>> Gitweb:     http://git.kernel.org/tip/0333a209cbf600e980fc55c24878a56f25f48b65
>> Author:     Andy Lutomirski <luto@kernel.org>
>> AuthorDate: Fri, 3 Jul 2015 12:44:34 -0700
>> Committer:  Ingo Molnar <mingo@kernel.org>
>> CommitDate: Tue, 7 Jul 2015 10:59:10 +0200
>>
>> x86/irq, context_tracking: Document how IRQ context tracking works and add an RCU assertion
>>
>> Signed-off-by: Andy Lutomirski <luto@kernel.org>
>> Cc: Andy Lutomirski <luto@amacapital.net>
>> Cc: Borislav Petkov <bp@alien8.de>
>> Cc: Brian Gerst <brgerst@gmail.com>
>> Cc: Denys Vlasenko <dvlasenk@redhat.com>
>> Cc: Denys Vlasenko <vda.linux@googlemail.com>
>> Cc: Frederic Weisbecker <fweisbec@gmail.com>
>> Cc: H. Peter Anvin <hpa@zytor.com>
>> Cc: Kees Cook <keescook@chromium.org>
>> Cc: Linus Torvalds <torvalds@linux-foundation.org>
>> Cc: Oleg Nesterov <oleg@redhat.com>
>> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>> Cc: Peter Zijlstra <peterz@infradead.org>
>> Cc: Rik van Riel <riel@redhat.com>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Cc: paulmck@linux.vnet.ibm.com
>> Link: http://lkml.kernel.org/r/e8bdc4ed0193fb2fd130f3d6b7b8023e2ec1ab62.1435952415.git.luto@kernel.org
>> Signed-off-by: Ingo Molnar <mingo@kernel.org>
>> ---
>>  arch/x86/kernel/irq.c | 15 +++++++++++++++
>>  1 file changed, 15 insertions(+)
>>
>> diff --git a/arch/x86/kernel/irq.c b/arch/x86/kernel/irq.c
>> index 88b36648..6233de0 100644
>> --- a/arch/x86/kernel/irq.c
>> +++ b/arch/x86/kernel/irq.c
>> @@ -216,8 +216,23 @@ __visible unsigned int __irq_entry do_IRQ(struct pt_regs *regs)
>>       unsigned vector = ~regs->orig_ax;
>>       unsigned irq;
>>
>> +     /*
>> +      * NB: Unlike exception entries, IRQ entries do not reliably
>> +      * handle context tracking in the low-level entry code.  This is
>> +      * because syscall entries execute briefly with IRQs on before
>> +      * updating context tracking state, so we can take an IRQ from
>> +      * kernel mode with CONTEXT_USER.  The low-level entry code only
>> +      * updates the context if we came from user mode, so we won't
>> +      * switch to CONTEXT_KERNEL.  We'll fix that once the syscall
>> +      * code is cleaned up enough that we can cleanly defer enabling
>> +      * IRQs.
>> +      */
>> +
>
> Now is it a problem to take interrupts in kernel mode with CONTEXT_USER?
> I'm not sure it's worth trying to make it not happen.

It's not currently a problem, but it would be nice if we could do the
equivalent of:

if (user_mode(regs)) {
  user_exit();  (or enter_from_user_mode or whatever)
} else {
  // don't bother -- already in CONTEXT_KERNEL
}

i.e. the same thing that do_general_protection, etc do in -tip.  That
would get rid of any need to store the previous context.

Currently we can't because of syscalls and maybe because of KVM.  KVM
has a weird fake interrupt thing.

>
>>       entering_irq();
>>
>> +     /* entering_irq() tells RCU that we're not quiescent.  Check it. */
>> +     rcu_lockdep_assert(rcu_is_watching(), "IRQ failed to wake up RCU");
>
> Why do we need to check that?

Sanity check.  If we're changing a bunch of context tracking details,
I want to assert that it actually works.

--Andy