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

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Message-ID: <alpine.DEB.2.11.1603172113470.3978@nanos>
Date:	Thu, 17 Mar 2016 21:20:51 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jens Axboe <axboe@...nel.dk>
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 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 ...

> > 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?

Thanks,

	tglx