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Message-ID: <20141121233834.GW5050@linux.vnet.ibm.com> Date: Fri, 21 Nov 2014 15:38:34 -0800 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Andy Lutomirski <luto@...capital.net> 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 03:06:48PM -0800, Andy Lutomirski wrote: > 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. 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@...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 > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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