linux-kernel - Re: [PATCHv3 0/2] arm64: fix hotplug rwsem boot fallout

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Date:   Thu, 11 May 2017 13:54:31 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LAK <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>, will.deacon@....com,
        catalin.marinas@....com, Sebastian Sewior <bigeasy@...utronix.de>,
        jbaron@...mai.com, Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>, suzuki.poulose@....com
Subject: Re: [PATCHv3 0/2] arm64: fix hotplug rwsem boot fallout

On Thu, May 11, 2017 at 12:01:21PM +0200, Thomas Gleixner wrote:
> On Thu, 11 May 2017, Mark Rutland wrote:
> > On Thu, May 11, 2017 at 10:30:39AM +0200, Thomas Gleixner wrote:
> > > On Wed, 10 May 2017, Thomas Gleixner wrote:
> > > secondary_start_kernel()
> > >   check_local_cpu_capabilities()
> > >     update_cpu_errata_workarounds()
> > >       update_cpu_capabilities()
> > >         static_key_enable()
> > > 	  __static_key_slow_inc()
> > > 	    jump_label_lock()
> > >        	      mutex_lock(&jump_label_mutex);
> > > 
> > > How is that supposed to work?
> > > 
> > > That call path is the low level CPU bringup, running in the context of the
> > > idle task of that CPU with interrupts and preemption disabled. Taking a
> > > mutex in that context, even if in that case the mutex is uncontended, is a
> > > NONO.

> > As an aside, do we have anything that should detect the broken mutex
> > usage? I've been testing kernels with LOCKDEP, PROVE_LOCKING,
> > DEBUG_ATOMIC_SLEEP, and friends, and nothing has complained so far.
> 
> Peter and myself were wondering about that already. No idea why that
> doesn't yell at you.

AFAICT, this happens early enough that system_state is SYSTEM_BOOTING.
In ___might_sleep(), we see this and bail out without a splat.

Thanks,
Mark.