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

Paul E. McKenney
2014-11-21T22:55:16Z

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@linux.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@amacapital.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.

> 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.

Signed-off-by: Paul E. McKenney <paulmck@linux.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 */