linux-kernel - Re: [PATCH v6 2/4] dax/bus: Use guard(device) in sysfs attribute helpers

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Date: Fri, 15 Dec 2023 08:27:42 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Vishal Verma <vishal.l.verma@...el.com>
Cc: Dan Williams <dan.j.williams@...el.com>,
	Dave Jiang <dave.jiang@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oscar Salvador <osalvador@...e.de>, linux-kernel@...r.kernel.org,
	nvdimm@...ts.linux.dev, linux-cxl@...r.kernel.org,
	David Hildenbrand <david@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Huang Ying <ying.huang@...el.com>, linux-mm@...ck.org,
	Joao Martins <joao.m.martins@...cle.com>
Subject: Re: [PATCH v6 2/4] dax/bus: Use guard(device) in sysfs attribute
 helpers

On Thu, Dec 14, 2023 at 10:25:27PM -0700, Vishal Verma wrote:
> Use the guard(device) macro to lock a 'struct device', and unlock it
> automatically when going out of scope using Scope Based Resource
> Management semantics. A lot of the sysfs attribute writes in
> drivers/dax/bus.c benefit from a cleanup using these, so change these
> where applicable.

Wait, why are you needing to call device_lock() at all here?  Why is dax
special in needing this when no other subsystem requires it?

> 
> Cc: Joao Martins <joao.m.martins@...cle.com>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Signed-off-by: Vishal Verma <vishal.l.verma@...el.com>
> ---
>  drivers/dax/bus.c | 143 ++++++++++++++++++++++--------------------------------
>  1 file changed, 59 insertions(+), 84 deletions(-)
> 
> diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
> index 1ff1ab5fa105..6226de131d17 100644
> --- a/drivers/dax/bus.c
> +++ b/drivers/dax/bus.c
> @@ -294,13 +294,10 @@ static ssize_t available_size_show(struct device *dev,
>  		struct device_attribute *attr, char *buf)
>  {
>  	struct dax_region *dax_region = dev_get_drvdata(dev);
> -	unsigned long long size;
>  
> -	device_lock(dev);
> -	size = dax_region_avail_size(dax_region);
> -	device_unlock(dev);
> +	guard(device)(dev);

You have a valid device here, why are you locking it?  How can it go
away?  And if it can, shouldn't you have a local lock for it, and not
abuse the driver core lock?

>  
> -	return sprintf(buf, "%llu\n", size);
> +	return sprintf(buf, "%llu\n", dax_region_avail_size(dax_region));

sysfs_emit() everywhere please.

But again, the issue is "why do you need a lock"?

thanks,

greg k-h