From mboxrd@z Thu Jan  1 00:00:00 1970
From: Chris Mason <chris.mason@oracle.com>
Subject: Re: [PATCH 3/3] btrfs: extended inode refs
Date: Thu, 10 May 2012 09:35:09 -0400
Message-ID: <20120510133509.GB11859@shiny>
References: <1333656543-4843-1-git-send-email-mfasheh@suse.de>
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 <4F871808.3050309@jan-o-sch.net>
 <20120508225739.GW17950@wotan.suse.de>
 <20120509170228.GA4025@shiny>
 <4FAB7B00.4090302@jan-o-sch.net>
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Cc: Mark Fasheh <mfasheh@suse.de>, linux-btrfs@vger.kernel.org,
	Josef Bacik <josef@redhat.com>
To: Jan Schmidt <list.btrfs@jan-o-sch.net>
Return-path: <linux-btrfs-owner@vger.kernel.org>
In-Reply-To: <4FAB7B00.4090302@jan-o-sch.net>
List-ID: <linux-btrfs.vger.kernel.org>

On Thu, May 10, 2012 at 10:23:28AM +0200, Jan Schmidt wrote:
> On Wed, May 09, 2012 at 19:02 (+0200), Chris Mason wrote:
> >>>> +
> >>>> +	while (1) {
> >>>> +		ret = btrfs_find_one_extref(fs_root, inum, offset, path, &iref2,
> >>>> +					    &offset);
> >>>> +		if (ret < 0)
> >>>> +			break;
> >>>> +		if (ret) {
> >>>> +			ret = found ? 0 : -ENOENT;
> >>>> +			break;
> >>>> +		}
> >>>> +		++found;
> >>>> +
> >>>> +		slot = path->slots[0];
> >>>> +		eb = path->nodes[0];
> >>>> +		/* make sure we can use eb after releasing the path */
> >>>> +		atomic_inc(&eb->refs);
> >>>
> >>> You need a blocking read lock here, too. Grab it before releasing the path.
> > 
> > If you're calling btrfs_search_slot, it will give you a blocking lock
> > on the leaf.  If you set path->leave_spinning before the call, you'll
> > have a spinning lock on the leaf.
> > 
> > If you unlock a block that you got from a path (like eb =
> > path->nodes[0]), the path structure has a flag for each level that
> > indicates if that block was locked or not.  See btrfs_release_path().
> > So, don't fiddle the locks without fiddling the paths.
> > 
> > You can switch from spinning to/from blocking without touching the path, it
> > figures that out.
> 
> Note that we're releasing the path shortly after. My suggestion was to
> grab an *additional* read lock after atomic_inc(&eb->refs) (probably
> better extent_buffer_get(eb)) and before btrfs_path_release().
> 
> An alternative would be to set path->locks[0] to NULL, which would
> btrfs_release_path prevent from unlocking it, kidnapping its lock for
> our purpose. But I much prefer the open coded solution.

Thanks, I see now.

-chris