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

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Message-ID: <89ea0a46cd1cf22393c6b05a50cd3e719ab6a4ba.camel@intel.com>
Date: Fri, 15 Dec 2023 17:32:50 +0000
From: "Verma, Vishal L" <vishal.l.verma@...el.com>
To: "Williams, Dan J" <dan.j.williams@...el.com>, "gregkh@...uxfoundation.org"
	<gregkh@...uxfoundation.org>
CC: "david@...hat.com" <david@...hat.com>, "joao.m.martins@...cle.com"
	<joao.m.martins@...cle.com>, "Jiang, Dave" <dave.jiang@...el.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>, "osalvador@...e.de"
	<osalvador@...e.de>, "linux-cxl@...r.kernel.org" <linux-cxl@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>, "Huang, Ying"
	<ying.huang@...el.com>, "nvdimm@...ts.linux.dev" <nvdimm@...ts.linux.dev>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH v6 2/4] dax/bus: Use guard(device) in sysfs attribute
 helpers

On Fri, 2023-12-15 at 09:15 -0800, Dan Williams wrote:
> 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.

I see why _a_ lock is needed both here and in size_show() - the size
calculations do a walk over discontiguous ranges, and we don't want the
device to get reconfigured in the middle of that. A different local
lock seems reasonable - however can that go as a separate cleanup that
stands on its own?

For this series, I'll add a cleanup to replace the sprintfs with
sysfs_emit().