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Subject: Re: [PATCH 1/2] bus: arm-ccn: Fix use of smp_processor_id() in
 preemptible context
To: Mark Rutland <mark.rutland@arm.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Pawel Moll <pawel.moll@arm.com>, linux-kernel@vger.kernel.org
References: <20170904173041.14119-1-marc.zyngier@arm.com>
 <20170904173041.14119-2-marc.zyngier@arm.com>
 <20170904173748.GH26317@leverpostej>
From: Marc Zyngier <marc.zyngier@arm.com>
Organization: ARM Ltd
Message-ID: <26ed6ba5-12ee-909c-4dce-bf84f0620252@arm.com>
Date: Mon, 4 Sep 2017 18:59:40 +0100
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On 04/09/17 18:37, Mark Rutland wrote:
> On Mon, Sep 04, 2017 at 06:30:40PM +0100, Marc Zyngier wrote:
>> 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. Turning this
>> into a get_cpu()/put_cpu() does the trick.
>>
>> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
>> ---
>>  drivers/bus/arm-ccn.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/bus/arm-ccn.c b/drivers/bus/arm-ccn.c
>> index e8c6946fed9d..8502f1767024 100644
>> --- a/drivers/bus/arm-ccn.c
>> +++ b/drivers/bus/arm-ccn.c
>> @@ -1297,7 +1297,8 @@ 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);
>> +	put_cpu();
> 
> I think this isn't quite right, and we need to keep a CPU online until
> we've registerd the hotplug callbacks. Otherwise, we could race with a
> concurrent hotplug, and not migrate events appropriately.
Right, I see. I'll respin that.

Thanks,

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