Date:	Fri, 21 Nov 2014 15:06:48 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc:	Borislav Petkov <bp@...en8.de>, X86 ML <x86@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Tony Luck <tony.luck@...el.com>,
	Andi Kleen <andi@...stfloor.org>,
	Josh Triplett <josh@...htriplett.org>,
	Frédéric Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH v4 2/5] x86, traps: Track entry into and exit from IST context

On Fri, Nov 21, 2014 at 2:55 PM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
> On Fri, Nov 21, 2014 at 02:19:17PM -0800, Andy Lutomirski wrote:
>> On Fri, Nov 21, 2014 at 2:07 PM, Paul E. McKenney
>> <paulmck@...ux.vnet.ibm.com> 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@...capital.net> 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.
>
> As far as I know, there are not.  If there are, someone needs to let
> me know!  ;-)
>
>> There are, however, machine checks in the idle loop, and maybe kprobes
>> (haven't checked), so I think this patch might fix real bugs.
>
> If you can get ISTs from the idle loop, then the patch is needed.
>
>> > 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.
>
> It currently does not, please see below patch.  Are you able to test
> nesting?  It would be really cool if you could do so -- I have no
> way to test this patch.

I can try.  It's sort of easy -- I'll put an int3 into do_nmi and add
a fixup to avoid crashing.

What should I look for?  Should I try to force full nohz on and assert
something?  I don't really know how to make full nohz work.

>
>> 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?
>
> If I understand correctly, this will result in context tracking invoking
> rcu_user_enter(), which will result in the rcu_dynticks counter having an
> odd value.  In that case, rcu_nmi_enter() will notice that RCU is already
> paying attention to this CPU via its check of atomic_read(&rdtp->dynticks)
> & 0x1), and will thus just return.  The matching rcu_nmi_exit() will
> notice that the nesting count is zero, and will also just return.
>
> Thus, everything works in that case.
>
> In contrast, if rcu_nmi_enter() was invoked from the idle loop, it
> would see that RCU is not paying attention to this CPU and that the
> NMI nesting depth (which rcu_nmi_enter() increments) used to be zero.
> It would then atomically increment rtdp->dynticks, forcing RCU to start
> paying attention to this CPU.  The matching rcu_nmi_exit() will see
> that the nesting count was non-zero, but became zero when decremented.
> This will cause rcu_nmi_exit() to atomically increment rtdp->dynticks,
> which will tell RCU to stop paying attention to this CPU.
>
>                                                         Thanx, Paul
>
> ------------------------------------------------------------------------
>
> rcu: Make rcu_nmi_enter() handle nesting
>
> Andy Lutomirski is introducing ISTs into x86, which from RCU's
> viewpoint are NMIs.  Because ISTs and NMIs can nest, rcu_nmi_enter()
> and rcu_nmi_exit() must now correctly handle nesting.  As luck would
> have it, rcu_nmi_exit() handles nesting but rcu_nmi_enter() does not.
> This patch therefore makes rcu_nmi_enter() handle nesting.

Thanks.  Should I add this to v5 of my series?

--Andy

>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 8749f43f3f05..875421aff6e3 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -770,7 +770,8 @@ void rcu_nmi_enter(void)
>         if (rdtp->dynticks_nmi_nesting == 0 &&
>             (atomic_read(&rdtp->dynticks) & 0x1))
>                 return;
> -       rdtp->dynticks_nmi_nesting++;
> +       if (rdtp->dynticks_nmi_nesting++ != 0)
> +               return; /* Nested NMI/IST/whatever. */
>         smp_mb__before_atomic();  /* Force delay from prior write. */
>         atomic_inc(&rdtp->dynticks);
>         /* CPUs seeing atomic_inc() must see later RCU read-side crit sects */
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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