linux-kernel - [PATCH] irqchip/gicv3: silence noisy DEBUG_PER_CPU

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Message-Id: <1474306155-3303-1-git-send-email-james.morse@arm.com>
Date:   Mon, 19 Sep 2016 18:29:15 +0100
From:   James Morse <james.morse@....com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-arm-kernel@...ts.infradead.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>,
        Marc Zyngier <marc.zyngier@....com>,
        James Morse <james.morse@....com>
Subject: [PATCH] irqchip/gicv3: silence noisy DEBUG_PER_CPU_MAPS warning

gic_raise_softirq() walks the list of cpus using for_each_cpu(), it calls
gic_compute_target_list() which advances the iterator by the number of
CPUs in the cluster.

If gic_compute_target_list() reaches the last CPU it leaves the iterator
pointing at the last CPU. This means the next time round the for_each_cpu()
loop cpumask_next() will be called with an invalid CPU.

This triggers a warning when built with CONFIG_DEBUG_PER_CPU_MAPS:
[    3.077738] GICv3: CPU1: found redistributor 1 region 0:0x000000002f120000
[    3.077943] CPU1: Booted secondary processor [410fd0f0]
[    3.078542] ------------[ cut here ]------------
[    3.078746] WARNING: CPU: 1 PID: 0 at ../include/linux/cpumask.h:121 gic_raise_softirq+0x12c/0x170
[    3.078812] Modules linked in:
[    3.078869]
[    3.078930] CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.8.0-rc5+ #5188
[    3.078994] Hardware name: Foundation-v8A (DT)
[    3.079059] task: ffff80087a1a0080 task.stack: ffff80087a19c000
[    3.079145] PC is at gic_raise_softirq+0x12c/0x170
[    3.079226] LR is at gic_raise_softirq+0xa4/0x170
[    3.079296] pc : [<ffff0000083ead24>] lr : [<ffff0000083eac9c>] pstate: 200001c9
[    3.081139] Call trace:
[    3.081202] Exception stack(0xffff80087a19fbe0 to 0xffff80087a19fd10)

[    3.082269] [<ffff0000083ead24>] gic_raise_softirq+0x12c/0x170
[    3.082354] [<ffff00000808e614>] smp_send_reschedule+0x34/0x40
[    3.082433] [<ffff0000080e80a0>] resched_curr+0x50/0x88
[    3.082512] [<ffff0000080e89d0>] check_preempt_curr+0x60/0xd0
[    3.082593] [<ffff0000080e8a60>] ttwu_do_wakeup+0x20/0xe8
[    3.082672] [<ffff0000080e8bb8>] ttwu_do_activate+0x90/0xc0
[    3.082753] [<ffff0000080ea9a4>] try_to_wake_up+0x224/0x370
[    3.082836] [<ffff0000080eabc8>] default_wake_function+0x10/0x18
[    3.082920] [<ffff000008103134>] __wake_up_common+0x5c/0xa0
[    3.083003] [<ffff0000081031f4>] __wake_up_locked+0x14/0x20
[    3.083086] [<ffff000008103f80>] complete+0x40/0x60
[    3.083168] [<ffff00000808df7c>] secondary_start_kernel+0x15c/0x1d0
[    3.083240] [<00000000808911a4>] 0x808911a4
[    3.113401] Detected PIPT I-cache on CPU2

Avoid updating the iterator if the next call to cpumask_next() would
cause the for_each_cpu() loop to exit.

There is no change to gic_raise_softirq()'s behaviour, (cpumask_next()s
eventual call to _find_next_bit() will return early as start >= nbits),
this patch just silences the warning.

Signed-off-by: James Morse <james.morse@....com>
---
 drivers/irqchip/irq-gic-v3.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/irqchip/irq-gic-v3.c b/drivers/irqchip/irq-gic-v3.c
index ede5672ab34d..9e37c447cc2c 100644
--- a/drivers/irqchip/irq-gic-v3.c
+++ b/drivers/irqchip/irq-gic-v3.c
@@ -548,6 +548,7 @@ static int gic_starting_cpu(unsigned int cpu)
 static u16 gic_compute_target_list(int *base_cpu, const struct cpumask *mask,
 				   unsigned long cluster_id)
 {
+	int next_cpu;
 	int cpu = *base_cpu;
 	unsigned long mpidr = cpu_logical_map(cpu);
 	u16 tlist = 0;
@@ -562,9 +563,10 @@ static u16 gic_compute_target_list(int *base_cpu, const struct cpumask *mask,
 
 		tlist |= 1 << (mpidr & 0xf);
 
-		cpu = cpumask_next(cpu, mask);
-		if (cpu >= nr_cpu_ids)
+		next_cpu = cpumask_next(cpu, mask);
+		if (next_cpu >= nr_cpu_ids)
 			goto out;
+		cpu = next_cpu;
 
 		mpidr = cpu_logical_map(cpu);
 
-- 
2.8.0.rc3