On Thu, Aug 28, 2014 at 04:54:25PM +0100, Daniel Thompson wrote:

On 28/08/14 16:01, Russell King - ARM Linux wrote:

quoted

On Tue, Aug 19, 2014 at 07:12:07PM +0100, Daniel Thompson wrote:

quoted

On 19/08/14 18:37, Russell King - ARM Linux wrote:

quoted

On Tue, Aug 19, 2014 at 05:45:53PM +0100, Daniel Thompson wrote:

quoted

+int register_fiq_nmi_notifier(struct notifier_block *nb)
+{
+	return atomic_notifier_chain_register(&fiq_nmi_chain, nb);
+}
+
+asmlinkage void __exception_irq_entry fiq_nmi_handler(struct pt_regs *regs)
+{
+	struct pt_regs *old_regs = set_irq_regs(regs);
+
+	nmi_enter();
+	atomic_notifier_call_chain(&fiq_nmi_chain, (unsigned long)regs, NULL);
+	nmi_exit();
+	set_irq_regs(old_regs);
+}

Really not happy with this.  What happens if a FIQ occurs while we're
inside register_fiq_nmi_notifier() - more specifically inside
atomic_notifier_chain_register() ?

Should depend on which side of the rcu update we're on.

I just asked Paul McKenney, our RCU expert... essentially, yes, RCU
stuff itself is safe in this context.  However, RCU stuff can call into
lockdep if lockdep is configured, and there are questions over lockdep.

Thanks for following this up.

I originally formed the opinion RCU was safe from FIQ because it is also
used to manage the NMI notification handlers for x86
(register_nmi_handler) and I understood the runtime constraints on FIQ
to be very similar.

Note that x86 manages the notifiers itself so it uses
list_for_each_entry_rcu() rather atomic_notifier_call_chain() but
nevertheless I think this boils down to the same thing w.r.t. safety
concerns.

quoted

There's some things which can be done to reduce the lockdep exposure
to it, such as ensuring that rcu_read_lock() is first called outside
of FIQ context.

lockdep is automatically disabled by calling nmi_enter() so all the
lockdep calls should end up following the early exit path based on
current->lockdep_recursion.

Ah, that was what I was missing.  Then the notification should be
safe from NMI, so have at it!  ;-)

							Thanx, Paul

quoted

There's concerns with whether either printk() in check_flags() could
be reached too (flags there should always indicate that IRQs were
disabled, so that reduces down to a question about just the first
printk() there.)

There's also the very_verbose() stuff for RCU lockdep classes which
Paul says must not be enabled.

Lastly, Paul isn't a lockdep expert, but he sees nothing that prevents
lockdep doing the deadlock checking as a result of the above call.

So... this coupled with my feeling that notifiers make it too easy for
unreviewed code to be hooked into this path, I'm fairly sure that we
don't want to be calling atomic notifier chains from FIQ context.