linux-kernel - Re: 4.5.0+ panic when setup loop device

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Date:	Thu, 17 Mar 2016 13:23:40 -0700
From:	Jens Axboe <axboe@...nel.dk>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Xiong Zhou <jencce.kernel@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
	Andreas Herrmann <aherrmann@...e.com>
Subject: Re: 4.5.0+ panic when setup loop device

On 03/17/2016 01:20 PM, Thomas Gleixner wrote:
> On Thu, 17 Mar 2016, Jens Axboe wrote:
>> On 03/17/2016 09:42 AM, Jens Axboe wrote:
>>> On 03/17/2016 05:01 AM, Thomas Gleixner wrote:
>>>> On Thu, 17 Mar 2016, Peter Zijlstra wrote:
>>>>> On Thu, Mar 17, 2016 at 12:39:46PM +0100, Thomas Gleixner wrote:
>>>>>> But we have to clarify and document whether holes in
>>>>>> cpu_possible_mask are not
>>>>>> allowed at all or if code like the above is simply broken.
>>>>>
>>>>> So the general rule is that cpumasks can have holes, and exempting one
>>>>> just muddles the water.
>>>>>
>>>>> Therefore I'd call the code just plain broken.
>>>>
>>>> Agreed.
>>>>
>>>> That macro is not really helping the readability of the code at all. So a
>>>> simple for_each_possible_cpu() loop would have avoided that wreckage.
>>>
>>> Does the attached work? The rest of blk-mq should deal with holes just
>
> Bah. Attachements ...

You'll live. Let's face it, all mailers suck in one way or another.

>>> fine, we found some of those issues on sparc. Not sure why this one
>>> slipped through the cracks.
>>
>> This might be better, we need to start at -1 to not miss the first one...
>> Still untested.
>
>> +static inline struct blk_mq_ctx *next_ctx(struct request_queue *q, int *i)
>> +{
>> +	do {
>> +		(*i)++;
>> +		if (*i < q->nr_queues) {
>> +			if (cpu_possible(*i))
>> +				return per_cpu_ptr(q->queue_ctx, *i);
>> +			continue;
>> +		}
>> +		break;
>> +	} while (1);
>> +
>> +	return NULL;
>> +}
>> +
>> +#define queue_for_each_ctx(q, ctx, i)					\
>> +	for ((i) = -1; (ctx = next_ctx((q), &(i))) != NULL;)
>> +
>
> What's wrong with
>
>          for_each_possible_cpu(cpu) {
>         		ctx = per_cpu_ptr(q->queue_ctx, cpu);
>
> 		....
> 	}
>
> instead of hiding it behind an incomprehensible macro mess?

We might not have mapped all of them.

-- 
Jens Axboe