linux-kernel - Re: [PATCH] hwmon: (fam15h_power) Disable preemption when reading registers

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Message-ID: <574EE1B3.9010704@roeck-us.net>
Date:	Wed, 1 Jun 2016 06:22:59 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Borislav Petkov <bp@...en8.de>, linux-hwmon@...r.kernel.org
Cc:	X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
	Rui Huang <ray.huang@....com>,
	Sherry Hurwitz <sherry.hurwitz@....com>
Subject: Re: [PATCH] hwmon: (fam15h_power) Disable preemption when reading
 registers

On 06/01/2016 03:04 AM, Borislav Petkov wrote:
> From: Borislav Petkov <bp@...e.de>
>
> We need to read a bunch of registers on each compute unit and possibly
> on the current CPU too. Disable preemption around it.
>

An explanation would be helpful. Is this a bug fix ? I would like to get
a confirmation from someone at AMD that this is really necessary.

Thanks,
Guenter

> Signed-off-by: Borislav Petkov <bp@...e.de>
> Cc: Rui Huang <ray.huang@....com>
> Cc: Sherry Hurwitz <sherry.hurwitz@....com>
> Cc: Guenter Roeck <linux@...ck-us.net>
> ---
>   drivers/hwmon/fam15h_power.c | 10 ++++++----
>   1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/hwmon/fam15h_power.c b/drivers/hwmon/fam15h_power.c
> index eb97a9241d17..69bb810f528b 100644
> --- a/drivers/hwmon/fam15h_power.c
> +++ b/drivers/hwmon/fam15h_power.c
> @@ -172,9 +172,9 @@ static void do_read_registers_on_cu(void *_data)
>    */
>   static int read_registers(struct fam15h_power_data *data)
>   {
> -	int this_cpu, ret, cpu;
>   	int core, this_core;
>   	cpumask_var_t mask;
> +	int ret, cpu;
>
>   	ret = zalloc_cpumask_var(&mask, GFP_KERNEL);
>   	if (!ret)
> @@ -183,7 +183,6 @@ static int read_registers(struct fam15h_power_data *data)
>   	memset(data->cu_on, 0, sizeof(int) * MAX_CUS);
>
>   	get_online_cpus();
> -	this_cpu = smp_processor_id();
>
>   	/*
>   	 * Choose the first online core of each compute unit, and then
> @@ -205,10 +204,13 @@ static int read_registers(struct fam15h_power_data *data)
>   		cpumask_set_cpu(cpumask_any(topology_sibling_cpumask(cpu)), mask);
>   	}
>
> -	if (cpumask_test_cpu(this_cpu, mask))
> +	preempt_disable();
> +	smp_call_function_many(mask, do_read_registers_on_cu, data, true);
> +
> +	if (cpumask_test_cpu(smp_processor_id(), mask))
>   		do_read_registers_on_cu(data);
>
> -	smp_call_function_many(mask, do_read_registers_on_cu, data, true);
> +	preempt_enable();
>   	put_online_cpus();
>
>   	free_cpumask_var(mask);
>