This is a note to let you know that I've just added the patch titled
btrfs: fix refcount_t usage when deleting btrfs_delayed_nodes
to the 4.14-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...
The filename of the patch is: btrfs-fix-refcount_t-usage-when-deleting-btrfs_delayed_nodes.patch and it can be found in the queue-4.14 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.
From ec35e48b286959991cdbb886f1bdeda4575c80b4 Mon Sep 17 00:00:00 2001
From: Chris Mason clm@fb.com Date: Fri, 15 Dec 2017 11:58:27 -0800 Subject: btrfs: fix refcount_t usage when deleting btrfs_delayed_nodes
From: Chris Mason clm@fb.com
commit ec35e48b286959991cdbb886f1bdeda4575c80b4 upstream.
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") 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 Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- fs/btrfs/delayed-inode.c | 45 ++++++++++++++++++++++++++++++++++----------- 1 file changed, 34 insertions(+), 11 deletions(-)
--- a/fs/btrfs/delayed-inode.c +++ b/fs/btrfs/delayed-inode.c @@ -87,6 +87,7 @@ static struct btrfs_delayed_node *btrfs_
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_ 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); } }
Patches currently in stable-queue which might be from clm@fb.com are
queue-4.14/btrfs-fix-refcount_t-usage-when-deleting-btrfs_delayed_nodes.patch