This is a note to let you know that I've just added the patch titled powerpc/smp: Call rcu_cpu_starting() earlier to the 5.9-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: powerpc-smp-call-rcu_cpu_starting-earlier.patch and it can be found in the queue-5.9 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From 99f070b62322a4b8c1252952735806d09eb44b68 Mon Sep 17 00:00:00 2001 From: Qian Cai <cai@xxxxxxxxxx> Date: Wed, 28 Oct 2020 14:23:34 -0400 Subject: powerpc/smp: Call rcu_cpu_starting() earlier From: Qian Cai <cai@xxxxxxxxxx> 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@xxxxxxxxxx> Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxx> [mpe: Add Fixes tag, reword slightly & expand change log] Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx> Link: https://lore.kernel.org/r/20201028182334.13466-1-cai@xxxxxxxxxx Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- 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); Patches currently in stable-queue which might be from cai@xxxxxxxxxx are queue-5.9/powerpc-smp-call-rcu_cpu_starting-earlier.patch