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

[Thomas Gleixner]

On Wed, 10 May 2017, Thomas Gleixner wrote:
> On Wed, 10 May 2017, Mark Rutland wrote:
> > [    0.182133] [<ffff200008155218>] lockdep_assert_hotplug_held+0x78/0x98
> > [    0.182161] [<ffff20000840a36c>] __static_key_slow_inc+0x174/0x2e0
> > [    0.182188] [<ffff20000840a654>] static_key_enable_cpuslocked+0x64/0xb0
> > [    0.182215] [<ffff2000080a1120>] update_cpu_capabilities+0x178/0x2d8
> > [    0.182243] [<ffff20000809e72c>] update_cpu_errata_workarounds_cpuslocked+0x1c/0x28
> > [    0.182270] [<ffff2000080a1420>] check_local_cpu_capabilities+0x1a0/0x248
> > [    0.182295] [<ffff2000080a2d18>] secondary_start_kernel+0x1e8/0x478
> > [    0.182317] [<000000008219a1b4>] 0x8219a1b4
> > [    0.182337] CPU features: enabling workaround for ARM erratum 834220
> > [    0.182362] ------------[ cut here ]------------
> > 
> > The problem is that the secondary CPU doesn't hold the rwsem when it
> > calls __static_key_slow_inc() in its boot path. It cannot take the
> > rwsem, since the primaary CPU holds this for the duration of onlining
> > the secondary CPU.

Looking deeper into that:

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.

Thanks,

	tglx