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SubjectRe: [PATCH 1/2] cpu/hotplug: Mute hotplug lockdep during init
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On 19/12/2018 18:23, Valentin Schneider wrote:
> Since we've had:
>
> commit cb538267ea1e ("jump_label/lockdep: Assert we hold the hotplug lock for _cpuslocked() operations")
>
> we've been getting some lockdep warnings during init, such as on HiKey960:
>
> [ 0.820495] WARNING: CPU: 4 PID: 0 at kernel/cpu.c:316 lockdep_assert_cpus_held+0x3c/0x48
> [ 0.820498] Modules linked in:
> [ 0.820509] CPU: 4 PID: 0 Comm: swapper/4 Tainted: G S 4.20.0-rc5-00051-g4cae42a #34
> [ 0.820511] Hardware name: HiKey960 (DT)
> [ 0.820516] pstate: 600001c5 (nZCv dAIF -PAN -UAO)
> [ 0.820520] pc : lockdep_assert_cpus_held+0x3c/0x48
> [ 0.820523] lr : lockdep_assert_cpus_held+0x38/0x48
> [ 0.820526] sp : ffff00000a9cbe50
> [ 0.820528] x29: ffff00000a9cbe50 x28: 0000000000000000
> [ 0.820533] x27: 00008000b69e5000 x26: ffff8000bff4cfe0
> [ 0.820537] x25: ffff000008ba69e0 x24: 0000000000000001
> [ 0.820541] x23: ffff000008fce000 x22: ffff000008ba70c8
> [ 0.820545] x21: 0000000000000001 x20: 0000000000000003
> [ 0.820548] x19: ffff00000a35d628 x18: ffffffffffffffff
> [ 0.820552] x17: 0000000000000000 x16: 0000000000000000
> [ 0.820556] x15: ffff00000958f848 x14: 455f3052464d4d34
> [ 0.820559] x13: 00000000769dde98 x12: ffff8000bf3f65a8
> [ 0.820564] x11: 0000000000000000 x10: ffff00000958f848
> [ 0.820567] x9 : ffff000009592000 x8 : ffff00000958f848
> [ 0.820571] x7 : ffff00000818ffa0 x6 : 0000000000000000
> [ 0.820574] x5 : 0000000000000000 x4 : 0000000000000001
> [ 0.820578] x3 : 0000000000000000 x2 : 0000000000000001
> [ 0.820582] x1 : 00000000ffffffff x0 : 0000000000000000
> [ 0.820587] Call trace:
> [ 0.820591] lockdep_assert_cpus_held+0x3c/0x48
> [ 0.820598] static_key_enable_cpuslocked+0x28/0xd0
> [ 0.820606] arch_timer_check_ool_workaround+0xe8/0x228
> [ 0.820610] arch_timer_starting_cpu+0xe4/0x2d8
> [ 0.820615] cpuhp_invoke_callback+0xe8/0xd08
> [ 0.820619] notify_cpu_starting+0x80/0xb8
> [ 0.820625] secondary_start_kernel+0x118/0x1d0
>
> We've also had a similar warning in sched_init_smp() for every
> asymmetric system that would enable the sched_asym_cpucapacity static
> key, although that was singled out in:
>
> commit 40fa3780bac2 ("sched/core: Take the hotplug lock in sched_init_smp()")
>
> Those warnings are actually harmless, since we cannot have hotplug
> operations at the time they appear. Instead of starting to sprinkle
> useless hotplug lock operations in the init codepaths, mute the
> warnings until they start warning about real problems.
>
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
> ---
> FYI Thomas Gleixner suggested using SYSTEM_SCHEDULING instead of
> SYSTEM_RUNNING, but that seems to still be too early - sched_init_smp()
> (and kernel_init() actually) hasn't completed yet, so we'd still get
> those warnings.
> ---
> kernel/cpu.c | 9 +++++++++
> 1 file changed, 9 insertions(+)
>
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 91d5c38..34e40ef 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -313,6 +313,15 @@ void cpus_write_unlock(void)
>
> void lockdep_assert_cpus_held(void)
> {
> + /*
> + * We can't have hotplug operations before userspace starts running,
> + * and some init codepaths will knowingly not take the hotplug lock.
> + * This is all valid, so mute lockdep until it makes sense to report
> + * unheld locks.
> + */
> + if (system_state < SYSTEM_RUNNING)
> + return;
> +
> percpu_rwsem_assert_held(&cpu_hotplug_lock);
> }
>
> --
> 2.7.4
>

FWIW: Acked-by: Marc Zyngier <marc.zyngier@arm.com>

Thanks for having kept an eye on it!

M.
--
Jazz is not dead. It just smells funny...

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