linux-kernel - Re: [PATCH 0/2] incorrect cpumask behavior with CPUMASK

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Message-ID: <87wq2zk5a4.fsf@rustcorp.com.au>
Date:	Mon, 02 Mar 2015 21:58:03 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Oleg Drokin <green@...uxhacker.ru>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"David S. Miller" <davem@...emloft.net>,
	"\<linux-kernel\@vger.kernel.org\> Mailing List" 
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/2] incorrect cpumask behavior with CPUMASK_OFFSTACK

Oleg Drokin <green@...uxhacker.ru> writes:
>>> The second patch that I am not sure if we wnat, but it seems to be useful
>>> until struct cpumask is fully dynamic is to convert what looks like
>>> whole-set operations e.g. copies, namely:
>>> cpumask_setall, cpumask_clear, cpumask_copy to always operate on NR_CPUS
>>> bits to ensure there's no stale garbage left in the mask should the
>>> cpu count increases later.
>> You can't do this, because dynamically allocated cpumasks don't have
>> NR_CPUS bits.
>
> Well, right now they certainly have. As in, it's a static define and we allocate
> a bitmap to fit the (in my case) up to 8192 bits into such off-stack masks.

You're right, we should fix that properly.  Right now, cpumask_size() has:

	/* FIXME: Once all cpumask assignments are eliminated, this
	 * can be nr_cpumask_bits */
	return BITS_TO_LONGS(NR_CPUS) * sizeof(long);

>> Let's just kill all the cpus_ functions.  This wasn't done originally
>> because archs which didn't care about offline cpumasks didn't want the
>> churn.  In particular, they must not copy struct cpumask by assignment,
>> and fixing those is a fair bit of churn.
>
> Ah, copy masks by assignment, I see.
> Well, I guess we can eliminate the users outside of the affected arch trees
> (I assume in x86 there would be no objections?) and perhaps add a warning to
> checkpatch.pl?

OK... I have done a sweep.  It's not *that* bad with spatch.

I'm going to remove all the old functions.  Expect a series RSN.

Cheers,
Rusty.

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