Re: [RFC PATCH] x86, mce: change the mce notifier to 'blocking' from 'atomic'

[Thomas Gleixner]

On Wed, 12 Apr 2017, Borislav Petkov wrote:

> On Wed, Apr 12, 2017 at 08:27:05PM +0000, Verma, Vishal L wrote:
> > But isn't the atomic notifier call chain always called in atomic
> > context?
> 
> No, it isn't. We're calling it in normal process context in
> mce_gen_pool_process() too.
> 
> So this early exit will avoid any sleeping in atomic context. And since
> there's nothing you can do about the errors reported in atomic context,
> we can actually use that fact.

No, you can't.

CONFIG_RCU_PREEMPT=n + CONFIG_PREEMPT_COUNT will disable preemption from
within __atomic_notifier_call_chain() via rcu_read_lock(). Ergo you wont
ever enter the handler.

The behaviour in the RCU code is inconsistent. CONFIG_RCU_PREEMPT=y does
obviouly not disable preemption, but it should still trigger the
might_sleep() check when a blocking function is called from within a rcu
read side critical section.

Thanks,

	tglx