linux-kernel - Re: [PATCH 1/3] padata: separate serial and parallel cpumasks

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Date:	Tue, 6 Jul 2010 12:31:21 +0400
From:	Dan Kruchinin <dkruchinin@....org>
To:	Steffen Klassert <steffen.klassert@...unet.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: [PATCH 1/3] padata: separate serial and parallel cpumasks

On Tue, Jul 6, 2010 at 11:46 AM, Steffen Klassert
<steffen.klassert@...unet.com> wrote:
> On Tue, Jul 06, 2010 at 11:40:11AM +0400, Dan Kruchinin wrote:
>> >> >
>> >>
>> >> I think we can use RCU anyway. For instance we could use a structure
>> >>
>> >> struct pcrypt_cpumask {
>> >>       cpumask_var_t           pmask;
>> >>       cpumask_var_t           smask;
>> >> };
>> >>
>> >> and add a pointer to a structure of that type to the instance context.
>> >> Then we could use this pointer for RCU and replace the whole structure
>> >> if a cpumask changes.
>>
>> But is pcrypt interested pmask? If it isn't, pmask field will be unused.
>>
>
> It's probaply not, in this case the struct could look like
>
> struct pcrypt_cpumask {
>        cpumask_var_t           smask;
> };
>

Would't it be the same as with a pointer to cpumask_var_t? I mean:
struct pcrypt {
  ...
  struct pcrypt_cpumask *mask;
  ...
} pencrypt;

To assign a pointer via RCU:

int cpumask_change_nitify(...) {
   ...
  struct pcrypt_cpumask *new_mask = kmalloc(sizeof(*mask), GFP);
  struct pcrypt_cpumask *old_mask = pencrypt.mask;

  if (!new_mask)
     error();
  if (!alloc_cpumask_var(&new_mask->smask, GFP_KERNEL))
     error();

  get_serial_cpumask_from_padata(new_mask->mask);
  rcu_assign_pointer(pencrypt.mask, new_mask);
  synchronize_rcu_bh();

  free_cpumask_var(old_mask->smask);
  kfree(old_mask);
  ...
}

It's a bit hard to read this code because at the first sight it
appears unclear and odd why we allocate the structure with only one
member.

-- 
W.B.R.
Dan Kruchinin
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