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

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Date:	Tue, 6 Jul 2010 15:28:25 +0200
From:	Steffen Klassert <steffen.klassert@...unet.com>
To:	Dan Kruchinin <dkruchinin@....org>
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 06, 2010 at 12:31:21PM +0400, Dan Kruchinin wrote:
> 
> Would't it be the same as with a pointer to cpumask_var_t? I mean:

Using a pointer to cpumask_var_t is a bit problematic because
you don't know a priori about the type of cpumask_var_t.
The type depends whether the cpumasks are on/off stack.
So the easiest thing is to embed it to a struct, then you don't
need to care about the type. If you allocate a struct of type
pcrypt_cpumask you get what you want to have.

> 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.
> 

We can easily add a code comment if this appears to be unclear :)
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