Messages in this thread |  | | Date | Thu, 11 May 2017 10:37:21 +0100 | From | Mark Rutland <> | Subject | Re: [PATCHv3 0/2] arm64: fix hotplug rwsem boot fallout |
| |
On Thu, May 11, 2017 at 10:30:39AM +0200, Thomas Gleixner wrote: > 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.
Urgh; good point. Thanks for taking a look.
I think I can solve both issues by deferring poking the keys, so I'll give that a go.
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.
Thanks, Mark.
|  |