Re: [Xen-devel] [PATCH] cleancache: constify cleancache_ops structure

[Julia Lawall <julia.lawall@xxxxxxx>]

On Thu, 21 Jan 2016, Rasmus Villemoes wrote:

> On Wed, Jan 20 2016, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> 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@xxxxxxx>
> >> 
> >> ---
> >> 
> >> 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...

I sent a corrected version this afternoon.

> 
> >>   */
> >> -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).

OK, in general, I have referred to the type rather than the structure name 
in these patches, since there can be more than one structure.

julia

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