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

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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

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[PATCHv3 0/2] arm64: fix hotplug rwsem boot fallout Mark Rutland <mark.rutland@arm.com> - 2017-04-28 13:10 +0200
  [PATCHv3 2/2] arm64: cpufeature: use static_branch_enable_cpuslocked() Mark Rutland <mark.rutland@arm.com> - 2017-04-28 13:10 +0200
    [tip:smp/hotplug] arm64/cpufeature: Use  static_branch_enable_cpuslocked() tip-bot for Mark Rutland <tipbot@zytor.com> - 2017-04-30 11:50 +0200
  [PATCHv3 1/2] jump_label: Provide static_key_[enable|/slow_inc]_cpuslocked() Mark Rutland <mark.rutland@arm.com> - 2017-04-28 13:10 +0200
    [tip:smp/hotplug] jump_label: Provide  static_key_[enable|/slow_inc]_cpuslocked() tip-bot for Sebastian Andrzej Siewior <tipbot@zytor.com> - 2017-04-30 11:50 +0200
  Re: [PATCHv3 0/2] arm64: fix hotplug rwsem boot fallout Mark Rutland <mark.rutland@arm.com> - 2017-05-10 20:20 +0200
    Re: [PATCHv3 0/2] arm64: fix hotplug rwsem boot fallout Thomas Gleixner <tglx@linutronix.de> - 2017-05-10 22:10 +0200
      Re: [PATCHv3 0/2] arm64: fix hotplug rwsem boot fallout Thomas Gleixner <tglx@linutronix.de> - 2017-05-11 10:40 +0200
        Re: [PATCHv3 0/2] arm64: fix hotplug rwsem boot fallout Mark Rutland <mark.rutland@arm.com> - 2017-05-11 11:40 +0200
          Re: [PATCHv3 0/2] arm64: fix hotplug rwsem boot fallout Thomas Gleixner <tglx@linutronix.de> - 2017-05-11 12:10 +0200
            Re: [PATCHv3 0/2] arm64: fix hotplug rwsem boot fallout Mark Rutland <mark.rutland@arm.com> - 2017-05-11 15:00 +0200

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