[PATCH] arm64: Use a raw spinlock in __install_bp_hardening_cb()

[will.deacon@arm.com (Will Deacon)]

On Tue, Nov 27, 2018 at 03:35:21PM +0000, James Morse wrote:
> __install_bp_hardening_cb() is called via stop_machine() as part
> of the cpu_enable callback. To force each CPU to take its turn
> when allocating slots, they take a spinlock.
> 
> With the RT patches applied, the spinlock becomes a mutex,
> and we get warnings about sleeping while in stop_machine():
> | [    0.319176] CPU features: detected: RAS Extension Support
> | [    0.319950] BUG: scheduling while atomic: migration/3/36/0x00000002
> | [    0.319955] Modules linked in:
> | [    0.319958] Preemption disabled at:
> | [    0.319969] [<ffff000008181ae4>] cpu_stopper_thread+0x7c/0x108
> | [    0.319973] CPU: 3 PID: 36 Comm: migration/3 Not tainted 4.19.1-rt3-00250-g330fc2c2a880 #2
> | [    0.319975] Hardware name: linux,dummy-virt (DT)
> | [    0.319976] Call trace:
> | [    0.319981]  dump_backtrace+0x0/0x148
> | [    0.319983]  show_stack+0x14/0x20
> | [    0.319987]  dump_stack+0x80/0xa4
> | [    0.319989]  __schedule_bug+0x94/0xb0
> | [    0.319991]  __schedule+0x510/0x560
> | [    0.319992]  schedule+0x38/0xe8
> | [    0.319994]  rt_spin_lock_slowlock_locked+0xf0/0x278
> | [    0.319996]  rt_spin_lock_slowlock+0x5c/0x90
> | [    0.319998]  rt_spin_lock+0x54/0x58
> | [    0.320000]  enable_smccc_arch_workaround_1+0xdc/0x260
> | [    0.320001]  __enable_cpu_capability+0x10/0x20
> | [    0.320003]  multi_cpu_stop+0x84/0x108
> | [    0.320004]  cpu_stopper_thread+0x84/0x108
> | [    0.320008]  smpboot_thread_fn+0x1e8/0x2b0
> | [    0.320009]  kthread+0x124/0x128
> | [    0.320010]  ret_from_fork+0x10/0x18
> 
> Switch this to a raw spinlock, as we know this is only called with
> IRQs masked.

There might be a lock-free way to do this, but this isn't a fast-path
so I've queued up your patch for 4.21.

Thanks!

Will