From: Qian Cai cai@redhat.com
commit 99f070b62322a4b8c1252952735806d09eb44b68 upstream.
The call to rcu_cpu_starting() in start_secondary() is not early enough in the CPU-hotplug onlining process, which results in lockdep splats as follows (with CONFIG_PROVE_RCU_LIST=y):
WARNING: suspicious RCU usage ----------------------------- kernel/locking/lockdep.c:3497 RCU-list traversed in non-reader section!!
other info that might help us debug this:
RCU used illegally from offline CPU! rcu_scheduler_active = 1, debug_locks = 1 no locks held by swapper/1/0.
Call Trace: dump_stack+0xec/0x144 (unreliable) lockdep_rcu_suspicious+0x128/0x14c __lock_acquire+0x1060/0x1c60 lock_acquire+0x140/0x5f0 _raw_spin_lock_irqsave+0x64/0xb0 clockevents_register_device+0x74/0x270 register_decrementer_clockevent+0x94/0x110 start_secondary+0x134/0x800 start_secondary_prolog+0x10/0x14
This is avoided by adding a call to rcu_cpu_starting() near the beginning of the start_secondary() function. Note that the raw_smp_processor_id() is required in order to avoid calling into lockdep before RCU has declared the CPU to be watched for readers.
It's safe to call rcu_cpu_starting() in the arch code as well as later in generic code, as explained by Paul:
It uses a per-CPU variable so that RCU pays attention only to the first call to rcu_cpu_starting() if there is more than one of them. This is even intentional, due to there being a generic arch-independent call to rcu_cpu_starting() in notify_cpu_starting().
So multiple calls to rcu_cpu_starting() are fine by design.
Fixes: 4d004099a668 ("lockdep: Fix lockdep recursion") Signed-off-by: Qian Cai cai@redhat.com Acked-by: Paul E. McKenney paulmck@kernel.org [mpe: Add Fixes tag, reword slightly & expand change log] Signed-off-by: Michael Ellerman mpe@ellerman.id.au Link: https://lore.kernel.org/r/20201028182334.13466-1-cai@redhat.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- arch/powerpc/kernel/smp.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/arch/powerpc/kernel/smp.c +++ b/arch/powerpc/kernel/smp.c @@ -1251,7 +1251,7 @@ static bool shared_caches; /* Activate a secondary processor. */ void start_secondary(void *unused) { - unsigned int cpu = smp_processor_id(); + unsigned int cpu = raw_smp_processor_id(); struct cpumask *(*sibling_mask)(int) = cpu_sibling_mask;
mmgrab(&init_mm);