linux-kernel - Re: [PATCH] cleancache: constify cleancache

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Date:	Thu, 21 Jan 2016 21:00:09 +0100
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:	Julia Lawall <Julia.Lawall@...6.fr>, linux-mm@...ck.org,
	kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>,
	xen-devel@...ts.xenproject.org
Subject: Re: [PATCH] cleancache: constify cleancache_ops structure

On Wed, Jan 20 2016, Konrad Rzeszutek Wilk <konrad.wilk@...cle.com> wrote:

> On Wed, Dec 23, 2015 at 10:06:24PM +0100, Julia Lawall wrote:
>> The cleancache_ops structure is never modified, so declare it as const.
>> 
>> This also removes the __read_mostly declaration on the cleancache_ops
>> variable declaration, since it seems redundant with const.
>> 
>> Done with the help of Coccinelle.
>> 
>> Signed-off-by: Julia Lawall <Julia.Lawall@...6.fr>
>> 
>> ---
>> 
>> Not sure that the __read_mostly change is correct.  Does it apply to the
>> variable, or to what the variable points to?
>
> It should just put the structure in the right section (.rodata).
>
> Thanks for the patch!

The __read_mostly marker should probably be left there...

>>   */
>> -static struct cleancache_ops *cleancache_ops __read_mostly;
>> +static const struct cleancache_ops *cleancache_ops;
>>  
>>  /*
>>   * Counters available via /sys/kernel/debug/cleancache (if debugfs is
>> @@ -49,7 +49,7 @@ static void cleancache_register_ops_sb(struct super_block *sb, void *unused)
>>  /*
>>   * Register operations for cleancache. Returns 0 on success.
>>   */
>> -int cleancache_register_ops(struct cleancache_ops *ops)
>> +int cleancache_register_ops(const struct cleancache_ops *ops)
>>  {
>>  	if (cmpxchg(&cleancache_ops, NULL, ops))
>>  		return -EBUSY;
>>

I don't know this code, but I assume that this is mostly a one-time
thing, so once cleancache_ops gets its value assigned, it doesn't
change, and that's what the __read_mostly is about (it applies to the
object declared, not whatever it happens to point to).

(Also, the commit message is slightly inaccurate: it is
tmem_cleancache_ops which is never changed and hence declared const;
changing the various pointers to it to const is just a necessary followup).

Rasmus