Messages in this thread | | | Date | Wed, 16 Oct 2013 09:37:12 -0400 | From | Steven Rostedt <> | Subject | Re: [PATCH] x86: Remove WARN_ON(in_nmi()) from vmalloc_fault |
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On Wed, 16 Oct 2013 15:28:15 +0200 Frederic Weisbecker <fweisbec@gmail.com> wrote:
> On Wed, Oct 16, 2013 at 09:14:37AM -0400, Steven Rostedt wrote: > > On Wed, 16 Oct 2013 15:08:57 +0200 > > Frederic Weisbecker <fweisbec@gmail.com> wrote: > > > > > > > Faults can call rcu_user_exit() / rcu_user_enter(). This is not supposed to happen > > > between rcu_nmi_enter() and rcu_nmi_exit(). rdtp->dynticks would be incremented in the > > > wrong way. > > > > > > Ah but we have an in_interrupt() check in context_tracking_user_enter() that protects > > > us against that. > > > > I will say that we should probably warn if it's any fault other than a > > vmalloc fault. A vmalloc fault should only happen in kernel space, and > > should not be happening from user code. > > The NMI can interrupt userspace. When the fault happens, it sees that context tracking > state is set to userspace (NMIs and interrupts in general don't exit that state, hence > the in_interrupt() check that returns when user_exit/enter is called) so it calls user_enter(). > But anyway we should be protected against that.
IIRC, NMI itself is safe to use rcu_read_lock(), at least I remember Paul making sure that stuff was lockless and NMI safe.
> > The WARN_ON() that I removed is from vmalloc fault. I don't see an > > issue with NMIs faulting via vmalloc. For any other page fault, sure, I > > would be concerned about it. But what's wrong with an NMI running > > module code? > > I won't argue further as none of us is going to change his opinion on this :)
Sure sure, yet another argument continues with two sides stubbornly refusing to negotiate about a looming future (de)fault!
-- Steve
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