From e82407d24968531d636fc5d544715c86675f62e6 Mon Sep 17 00:00:00 2001
From: Nikolay Borisov <nborisov@suse.com>
Date: Mon, 8 Feb 2021 10:26:54 +0200
Subject: [PATCH] btrfs: fix race between extent freeing/allocation when using
 bitmaps

commit 3c17916510428dbccdf657de050c34e208347089 upstream.

During allocation the allocator will try to allocate an extent using
cluster policy. Once the current cluster is exhausted it will remove the
entry under btrfs_free_cluster::lock and subsequently acquire
btrfs_free_space_ctl::tree_lock to dispose of the already-deleted entry
and adjust btrfs_free_space_ctl::total_bitmap. This poses a problem
because there exists a race condition between removing the entry under
one lock and doing the necessary accounting holding a different lock
since extent freeing only uses the 2nd lock. This can result in the
following situation:

T1:                                    T2:
btrfs_alloc_from_cluster               insert_into_bitmap <holds tree_lock>
 if (entry->bytes == 0)                   if (block_group && !list_empty(&block_group->cluster_list)) {
    rb_erase(entry)

 spin_unlock(&cluster->lock);
   (total_bitmaps is still 4)           spin_lock(&cluster->lock);
                                         <doesn't find entry in cluster->root>
 spin_lock(&ctl->tree_lock);             <goes to new_bitmap label, adds
<blocked since T2 holds tree_lock>       <a new entry and calls add_new_bitmap>
					    recalculate_thresholds  <crashes,
                                              due to total_bitmaps
					      becoming 5 and triggering
					      an ASSERT>

To fix this ensure that once depleted, the cluster entry is deleted when
both cluster lock and tree locks are held in the allocator (T1), this
ensures that even if there is a race with a concurrent
insert_into_bitmap call it will correctly find the entry in the cluster
and add the new space to it.

CC: <stable@vger.kernel.org> # 4.4+
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 fs/btrfs/free-space-cache.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c
index 9ebc8e2..ba28070 100644
--- a/fs/btrfs/free-space-cache.c
+++ b/fs/btrfs/free-space-cache.c
@@ -3034,8 +3034,6 @@ u64 btrfs_alloc_from_cluster(struct btrfs_block_group *block_group,
 			entry->bytes -= bytes;
 		}
 
-		if (entry->bytes == 0)
-			rb_erase(&entry->offset_index, &cluster->root);
 		break;
 	}
 out:
@@ -3052,7 +3050,10 @@ out:
 	ctl->free_space -= bytes;
 	if (!entry->bitmap && !btrfs_free_space_trimmed(entry))
 		ctl->discardable_bytes[BTRFS_STAT_CURR] -= bytes;
+
+	spin_lock(&cluster->lock);
 	if (entry->bytes == 0) {
+		rb_erase(&entry->offset_index, &cluster->root);
 		ctl->free_extents--;
 		if (entry->bitmap) {
 			kmem_cache_free(btrfs_free_space_bitmap_cachep,
@@ -3065,6 +3066,7 @@ out:
 		kmem_cache_free(btrfs_free_space_cachep, entry);
 	}
 
+	spin_unlock(&cluster->lock);
 	spin_unlock(&ctl->tree_lock);
 
 	return ret;
-- 
2.7.4