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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Fri, Nov 21, 2014 at 03:06:48PM -0800, Andy Lutomirski wrote:
> On Fri, Nov 21, 2014 at 2:55 PM, Paul E. McKenney
> <paulmck@linux.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@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.
> 
> 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.

You should look for the WARN_ON_ONCE() calls in rcu_nmi_enter() and
rcu_nmi_exit() to fire.

							Thanx, Paul

> >> 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@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 */
> >
> 
> 
> 
> -- 
> Andy Lutomirski
> AMA Capital Management, LLC
>