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btrfs: fix refcount_t usage when deleting btrfs_delayed_nodes

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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.

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.

Fixes: 6de5f18e7b0da ("btrfs: fix refcount_t usage when deleting btrfs_delayed_node")
CC: <stable@vger.kernel.org> # 4.12+
Signed-off-by: Chris Mason <clm@fb.com>
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Chris Mason 2017-12-15 11:58:27 -08:00 committed by David Sterba
parent beed9263f4
commit ec35e48b28
1 changed files with 34 additions and 11 deletions
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45
fs/btrfs/delayed-inode.c
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@ -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);
}
}