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Subject[PATCH v2 1/2] bus: arm-ccn: Fix use of smp_processor_id() in preemptible context
Date
Booting a DEBUG_PREEMPT enabled kernel on a CCN-based system
results in the following splat:

[...]
arm-ccn e8000000.ccn: No access to interrupts, using timer.
BUG: using smp_processor_id() in preemptible [00000000] code: swapper/0/1
caller is debug_smp_processor_id+0x1c/0x28
CPU: 1 PID: 1 Comm: swapper/0 Not tainted 4.13.0 #6111
Hardware name: AMD Seattle/Seattle, BIOS 17:08:23 Jun 26 2017
Call trace:
[<ffff000008089e78>] dump_backtrace+0x0/0x278
[<ffff00000808a22c>] show_stack+0x24/0x30
[<ffff000008bc3bc4>] dump_stack+0x8c/0xb0
[<ffff00000852b534>] check_preemption_disabled+0xfc/0x100
[<ffff00000852b554>] debug_smp_processor_id+0x1c/0x28
[<ffff000008551bd8>] arm_ccn_probe+0x358/0x4f0
[...]

as we use smp_processor_id() in the wrong context.

Turn this into a get_cpu()/put_cpu() that extends over the CPU hotplug
registration, making sure that we don't race against a CPU down operation.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
---
drivers/bus/arm-ccn.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/bus/arm-ccn.c b/drivers/bus/arm-ccn.c
index e8c6946fed9d..8614d7fa457e 100644
--- a/drivers/bus/arm-ccn.c
+++ b/drivers/bus/arm-ccn.c
@@ -1297,7 +1297,7 @@ static int arm_ccn_pmu_init(struct arm_ccn *ccn)
}

/* Pick one CPU which we will use to collect data from CCN... */
- cpumask_set_cpu(smp_processor_id(), &ccn->dt.cpu);
+ cpumask_set_cpu(get_cpu(), &ccn->dt.cpu);

/* Also make sure that the overflow interrupt is handled by this CPU */
if (ccn->irq) {
@@ -1314,10 +1314,12 @@ static int arm_ccn_pmu_init(struct arm_ccn *ccn)

cpuhp_state_add_instance_nocalls(CPUHP_AP_PERF_ARM_CCN_ONLINE,
&ccn->dt.node);
+ put_cpu();
return 0;

error_pmu_register:
error_set_affinity:
+ put_cpu();
ida_simple_remove(&arm_ccn_pmu_ida, ccn->dt.id);
for (i = 0; i < ccn->num_xps; i++)
writel(0, ccn->xp[i].base + CCN_XP_DT_CONTROL);
--
2.11.0
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