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

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Date:   Wed, 12 Apr 2017 23:50:45 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Borislav Petkov <bp@...e.de>
cc:     "Verma, Vishal L" <vishal.l.verma@...el.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...1.01.org>,
        "Luck, Tony" <tony.luck@...el.com>,
        "ross.zwisler@...ux.intel.com" <ross.zwisler@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH] x86, mce: change the mce notifier to 'blocking' from
 'atomic'

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