On Fri, Nov 21, 2014 at 2:07 PM, Paul E. McKenney <paul...@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 <l...@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. There are, however, machine checks in the idle loop, and maybe kprobes (haven't checked), so I think this patch might fix real bugs. > 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. 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? --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
