Re: [PATCH] btrfs: fix refcount_t usage when deleting btrfs_delayed_nodes

[Liu Bo <bo.li.liu@oracle.com>]

On Sat, Dec 16, 2017 at 08:42:51AM +0200, Nikolay Borisov wrote:
> 
> 
> On 15.12.2017 21:58, Chris Mason wrote:
> > refcounts have a generic implementation and an asm optimized one.  The
> > generic version has extra debugging to make sure that once a refcount
> > goes to zero, refcount_inc won't increase it.
>                   ^^^^^^^^
> 
> I guess you meant to say refcount_add

refcount_inc may also just throw a warning without bumping the refcnt.

Thanks,

-liubo
> > 
> > The btrfs delayed inode code wasn't expecting this, and we're tripping
> > over the warnings when the generic refcounts are used.  We ended up with
> > this race:
> > 
> > Process A                                         Process B
> >                                                   btrfs_get_delayed_node()
> > 						  spin_lock(root->inode_lock)
> > 						  radix_tree_lookup()
> > __btrfs_release_delayed_node()
> > refcount_dec_and_test(&delayed_node->refs)
> > our refcount is now zero
> > 						  refcount_add(2) <---
> > 						  warning here, refcount
> >                                                   unchanged
> > 
> > spin_lock(root->inode_lock)
> > radix_tree_delete()
> > 
> > With the generic refcounts, we actually warn again when process B above
> > tries to release his refcount because refcount_add() turned into a
> > no-op.
> > 
> > We saw this in production on older kernels without the asm optimized
> > refcounts.
> > 
> > The fix used here is to use refcount_inc_not_zero() to detect when the
> > object is in the middle of being freed and return NULL.  This is almost
> > always the right answer anyway, since we usually end up pitching the
> > delayed_node if it didn't have fresh data in it.
> > 
> > This also changes __btrfs_release_delayed_node() to remove the extra
> > check for zero refcounts before radix tree deletion.
> > btrfs_get_delayed_node() was the only path that was allowing refcounts
> > to go from zero to one.
> > 
> > Signed-off-by: Chris Mason <clm@fb.com>
> > Fixes: 6de5f18e7b0da
> > cc: <stable@vger.kernel.org> #4.12+
> > ---
> >  fs/btrfs/delayed-inode.c | 45 ++++++++++++++++++++++++++++++++++-----------
> >  1 file changed, 34 insertions(+), 11 deletions(-)
> > 
> > diff --git a/fs/btrfs/delayed-inode.c b/fs/btrfs/delayed-inode.c
> > index 5d73f79..84c54af 100644
> > --- a/fs/btrfs/delayed-inode.c
> > +++ b/fs/btrfs/delayed-inode.c
> > @@ -87,6 +87,7 @@ static struct btrfs_delayed_node *btrfs_get_delayed_node(
> >  
> >  	spin_lock(&root->inode_lock);
> >  	node = radix_tree_lookup(&root->delayed_nodes_tree, ino);
> > +
> >  	if (node) {
> >  		if (btrfs_inode->delayed_node) {
> >  			refcount_inc(&node->refs);	/* can be accessed */
> > @@ -94,9 +95,30 @@ static struct btrfs_delayed_node *btrfs_get_delayed_node(
> >  			spin_unlock(&root->inode_lock);
> >  			return node;
> >  		}
> > -		btrfs_inode->delayed_node = node;
> > -		/* can be accessed and cached in the inode */
> > -		refcount_add(2, &node->refs);
> > +
> > +		/* it's possible that we're racing into the middle of
> > +		 * removing this node from the radix tree.  In this case,
> > +		 * the refcount was zero and it should never go back
> > +		 * to one.  Just return NULL like it was never in the radix
> > +		 * at all; our release function is in the process of removing
> > +		 * it.
> > +		 *
> > +		 * Some implementations of refcount_inc refuse to
> > +		 * bump the refcount once it has hit zero.  If we don't do
> > +		 * this dance here, refcount_inc() may decide to
> > +		 * just WARN_ONCE() instead of actually bumping the refcount.
> > +		 *
> > +		 * If this node is properly in the radix, we want to
> > +		 * bump the refcount twice, once for the inode
> > +		 * and once for this get operation.
> > +		 */
> > +		if (refcount_inc_not_zero(&node->refs)) {
> > +			refcount_inc(&node->refs);
> > +			btrfs_inode->delayed_node = node;
> > +		} else {
> > +			node = NULL;
> > +		}
> > +
> >  		spin_unlock(&root->inode_lock);
> >  		return node;
> >  	}
> > @@ -254,17 +276,18 @@ static void __btrfs_release_delayed_node(
> >  	mutex_unlock(&delayed_node->mutex);
> >  
> >  	if (refcount_dec_and_test(&delayed_node->refs)) {
> > -		bool free = false;
> >  		struct btrfs_root *root = delayed_node->root;
> > +
> >  		spin_lock(&root->inode_lock);
> > -		if (refcount_read(&delayed_node->refs) == 0) {
> > -			radix_tree_delete(&root->delayed_nodes_tree,
> > -					  delayed_node->inode_id);
> > -			free = true;
> > -		}
> > +		/*
> > +		 * once our refcount goes to zero, nobody is allowed to
> > +		 * bump it back up.  We can delete it now
> > +		 */
> > +		ASSERT(refcount_read(&delayed_node->refs) == 0);
> > +		radix_tree_delete(&root->delayed_nodes_tree,
> > +				  delayed_node->inode_id);
> >  		spin_unlock(&root->inode_lock);
> > -		if (free)
> > -			kmem_cache_free(delayed_node_cache, delayed_node);
> > +		kmem_cache_free(delayed_node_cache, delayed_node);
> >  	}
> >  }
> >  
> > 
> --
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