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 09:15:45 -0800
From: Dan Williams <dan.j.williams@...el.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 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

Greg Kroah-Hartman wrote:
> 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?

Yes, this is a driver-core lock abuse written by someone who should have
known better. And yes, a local lock to protect the dax_region resource
tree should replace this. A new rwsem to synchronize all list walks
seems appropriate.