From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx0a-00082601.pphosted.com ([67.231.145.42]:36008 "EHLO mx0a-00082601.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755406AbdLOT63 (ORCPT ); Fri, 15 Dec 2017 14:58:29 -0500 Received: from pps.filterd (m0109333.ppops.net [127.0.0.1]) by mx0a-00082601.pphosted.com (8.16.0.21/8.16.0.21) with SMTP id vBFJsRGD032595 for ; Fri, 15 Dec 2017 11:58:28 -0800 Received: from mail.thefacebook.com ([199.201.64.23]) by mx0a-00082601.pphosted.com with ESMTP id 2evm2qr705-1 (version=TLSv1 cipher=ECDHE-RSA-AES256-SHA bits=256 verify=NOT) for ; Fri, 15 Dec 2017 11:58:28 -0800 From: Chris Mason To: CC: Subject: [PATCH] btrfs: fix refcount_t usage when deleting btrfs_delayed_nodes Date: Fri, 15 Dec 2017 11:58:27 -0800 Message-ID: <20171215195827.3952156-1-clm@fb.com> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-btrfs-owner@vger.kernel.org List-ID: 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. Signed-off-by: Chris Mason Fixes: 6de5f18e7b0da cc: #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); } } -- 2.9.5