Date: Wed, 16 Oct 2013 09:37:12 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        "H. Peter Anvin" <hpa@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "paulmck@linux.vnet.ibm.com" <paulmck@linux.vnet.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "x86@kernel.org" <x86@kernel.org>,
        "Wang, Xiaoming" <xiaoming.wang@intel.com>,
        "Li, Zhuangzhi" <zhuangzhi.li@intel.com>,
        "Liu, Chuansheng" <chuansheng.liu@intel.com>
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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