Re: [PATCH] irqchip/gicv3: silence noisy DEBUG_PER_CPU_MAPS warning

[Marc Zyngier]

Hi James,

On 19/09/16 18:29, James Morse wrote:
> 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@xxxxxxx>
> ---
>  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);
>  
> 

It took me a while to convince myself that the original code was
correct, and it obviously wasn't... Oh well, paper bag time.

Acked-by: Marc Zyngier <marc.zyngier@xxxxxxx>

Thomas, any chance you could pick that one for -rc8?

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...