linux-kernel - [PATCH] powerpc/smp: Move rcu_cpu

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Message-Id: <20201028182334.13466-1-cai@redhat.com>
Date:   Wed, 28 Oct 2020 14:23:34 -0400
From:   Qian Cai <cai@...hat.com>
To:     "Paul E . McKenney" <paulmck@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        Qian Cai <cai@...hat.com>
Subject: [PATCH] powerpc/smp: Move rcu_cpu_starting() earlier

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:

 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 moving the call to rcu_cpu_starting up 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.

Link: https://lore.kernel.org/lkml/160223032121.7002.1269740091547117869.tip-bot2@tip-bot2/
Signed-off-by: Qian Cai <cai@...hat.com>
---
 arch/powerpc/kernel/smp.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 3c6b9822f978..8c2857cbd960 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -1393,13 +1393,14 @@ static void add_cpu_to_masks(int cpu)
 /* Activate a secondary processor. */
 void start_secondary(void *unused)
 {
-	unsigned int cpu = smp_processor_id();
+	unsigned int cpu = raw_smp_processor_id();
 
 	mmgrab(&init_mm);
 	current->active_mm = &init_mm;
 
 	smp_store_cpu_info(cpu);
 	set_dec(tb_ticks_per_jiffy);
+	rcu_cpu_starting(cpu);
 	preempt_disable();
 	cpu_callin_map[cpu] = 1;
 
-- 
2.28.0