linux-kernel - Re: [PATCH v2 2/4] dax/bus.c: fix locking for unregister_dax_dev / unregister_dax

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Date: Tue, 30 Apr 2024 04:11:39 +0000
From: "Verma, Vishal L" <vishal.l.verma@...el.com>
To: "Williams, Dan J" <dan.j.williams@...el.com>, "Jiang, Dave"
	<dave.jiang@...el.com>, "Schofield, Alison" <alison.schofield@...el.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
CC: "linux-mm@...ck.org" <linux-mm@...ck.org>, "linux-cxl@...r.kernel.org"
	<linux-cxl@...r.kernel.org>, "nvdimm@...ts.linux.dev"
	<nvdimm@...ts.linux.dev>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/4] dax/bus.c: fix locking for unregister_dax_dev /
 unregister_dax_mapping paths

On Mon, 2024-04-29 at 18:25 -0700, Dan Williams wrote:
> Vishal Verma wrote:
> > Commit c05ae9d85b47 ("dax/bus.c: replace driver-core lock usage by a local rwsem")
> > was a bit overzealous in eliminating device_lock() usage, and ended up
> > removing a couple of lock acquisitions which were needed, and as a
> > result, fix some of the conditional locking missteps that the above
> > commit introduced in unregister_dax_dev() and unregister_dax_mapping().
> 
> I think it makes sense to tell the story a bit about why the
> delete_store() conversion was problematic, because the
> unregister_dev_dax() changes were just a knock-on effect to fixing the
> delete_store() flow.
> 
> Something like:
> 
> ---
> commit c05ae9d85b47 ("dax/bus.c: replace driver-core lock usage by a local rwsem")
> aimed to undo device_lock() abuses for protecting changes to dax-driver
> internal data-structures like the dax_region resource tree to
> device-dax-instance range structures. However, the device_lock() was legitamately
> enforcing that devices to be deleted were not current actively attached
> to any driver nor assigned any capacity from the region.
> ---
> 
> ...you can fill in a couple notes about the knock-on fixups after that
> was restored.

Sounds good, updated!

> 
> >  
> > @@ -560,15 +551,12 @@ static ssize_t delete_store(struct device *dev, struct device_attribute *attr,
> >  	if (!victim)
> >  		return -ENXIO;
> >  
> > -	rc = down_write_killable(&dax_region_rwsem);
> > -	if (rc)
> > -		return rc;
> > -	rc = down_write_killable(&dax_dev_rwsem);
> > -	if (rc) {
> > -		up_write(&dax_region_rwsem);
> > -		return rc;
> > -	}
> > +	device_lock(dev);
> > +	device_lock(victim);
> >  	dev_dax = to_dev_dax(victim);
> > +	rc = down_write_killable(&dax_dev_rwsem);
> 
> This begs the question, why down_write_killable(), but not
> device_lock_interruptible()?

Do you mean change the device_lock()s to device_lock_interruptible() in
addition to the taking the rwsem (i.e. not instead of the rwsem..)?
I guess I just restored what was there previously - but the
interruptible variant makes sense, I can make that change.

> 
> I do not expect any of this is long running so likely down_write() is
> sufficient here, especially since the heaviest locks to acquire are
> already held by the time rwsem is considered.
> 
> Other than that this looks good to me:
> 
> You can include my Reviewed-by on the next posting.

Thanks for the review Dan!