1 files changed, 382 insertions, 99 deletions

diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
index 38b127b9edfc..a3b830b8410a 100644
--- a/fs/btrfs/block-group.c
+++ b/fs/btrfs/block-group.c
@@ -1498,9 +1498,18 @@ void btrfs_reclaim_bgs_work(struct work_struct *work)
 	if (!btrfs_exclop_start(fs_info, BTRFS_EXCLOP_BALANCE))
 		return;
 
-	mutex_lock(&fs_info->reclaim_bgs_lock);
+	/*
+	 * Long running balances can keep us blocked here for eternity, so
+	 * simply skip reclaim if we're unable to get the mutex.
+	 */
+	if (!mutex_trylock(&fs_info->reclaim_bgs_lock)) {
+		btrfs_exclop_finish(fs_info);
+		return;
+	}
+
 	spin_lock(&fs_info->unused_bgs_lock);
 	while (!list_empty(&fs_info->reclaim_bgs)) {
+		u64 zone_unusable;
 		int ret = 0;
 
 		bg = list_first_entry(&fs_info->reclaim_bgs,
@@ -1534,16 +1543,25 @@ void btrfs_reclaim_bgs_work(struct work_struct *work)
 			goto next;
 		}
 
+		/*
+		 * Cache the zone_unusable value before turning the block group
+		 * to read only. As soon as the blog group is read only it's
+		 * zone_unusable value gets moved to the block group's read-only
+		 * bytes and isn't available for calculations anymore.
+		 */
+		zone_unusable = bg->zone_unusable;
 		ret = inc_block_group_ro(bg, 0);
 		up_write(&space_info->groups_sem);
 		if (ret < 0)
 			goto next;
 
-		btrfs_info(fs_info, "reclaiming chunk %llu with %llu%% used",
-				bg->start, div_u64(bg->used * 100, bg->length));
+		btrfs_info(fs_info,
+			"reclaiming chunk %llu with %llu%% used %llu%% unusable",
+				bg->start, div_u64(bg->used * 100, bg->length),
+				div64_u64(zone_unusable * 100, bg->length));
 		trace_btrfs_reclaim_block_group(bg);
 		ret = btrfs_relocate_chunk(fs_info, bg->start);
-		if (ret)
+		if (ret && ret != -EAGAIN)
 			btrfs_err(fs_info, "error relocating chunk %llu",
 				  bg->start);
 
@@ -2087,11 +2105,22 @@ static int fill_dummy_bgs(struct btrfs_fs_info *fs_info)
 		bg->used = em->len;
 		bg->flags = map->type;
 		ret = btrfs_add_block_group_cache(fs_info, bg);
+		/*
+		 * We may have some valid block group cache added already, in
+		 * that case we skip to the next one.
+		 */
+		if (ret == -EEXIST) {
+			ret = 0;
+			btrfs_put_block_group(bg);
+			continue;
+		}
+
 		if (ret) {
 			btrfs_remove_free_space_cache(bg);
 			btrfs_put_block_group(bg);
 			break;
 		}
+
 		btrfs_update_space_info(fs_info, bg->flags, em->len, em->len,
 					0, 0, &space_info);
 		bg->space_info = space_info;
@@ -2194,9 +2223,24 @@ int btrfs_read_block_groups(struct btrfs_fs_info *info)
 	ret = check_chunk_block_group_mappings(info);
 error:
 	btrfs_free_path(path);
+	/*
+	 * We've hit some error while reading the extent tree, and have
+	 * rescue=ibadroots mount option.
+	 * Try to fill the tree using dummy block groups so that the user can
+	 * continue to mount and grab their data.
+	 */
+	if (ret && btrfs_test_opt(info, IGNOREBADROOTS))
+		ret = fill_dummy_bgs(info);
 	return ret;
 }
 
+/*
+ * This function, insert_block_group_item(), belongs to the phase 2 of chunk
+ * allocation.
+ *
+ * See the comment at btrfs_chunk_alloc() for details about the chunk allocation
+ * phases.
+ */
 static int insert_block_group_item(struct btrfs_trans_handle *trans,
 				   struct btrfs_block_group *block_group)
 {
@@ -2219,15 +2263,108 @@ static int insert_block_group_item(struct btrfs_trans_handle *trans,
 	return btrfs_insert_item(trans, root, &key, &bgi, sizeof(bgi));
 }
 
+static int insert_dev_extent(struct btrfs_trans_handle *trans,
+			    struct btrfs_device *device, u64 chunk_offset,
+			    u64 start, u64 num_bytes)
+{
+	struct btrfs_fs_info *fs_info = device->fs_info;
+	struct btrfs_root *root = fs_info->dev_root;
+	struct btrfs_path *path;
+	struct btrfs_dev_extent *extent;
+	struct extent_buffer *leaf;
+	struct btrfs_key key;
+	int ret;
+
+	WARN_ON(!test_bit(BTRFS_DEV_STATE_IN_FS_METADATA, &device->dev_state));
+	WARN_ON(test_bit(BTRFS_DEV_STATE_REPLACE_TGT, &device->dev_state));
+	path = btrfs_alloc_path();
+	if (!path)
+		return -ENOMEM;
+
+	key.objectid = device->devid;
+	key.type = BTRFS_DEV_EXTENT_KEY;
+	key.offset = start;
+	ret = btrfs_insert_empty_item(trans, root, path, &key, sizeof(*extent));
+	if (ret)
+		goto out;
+
+	leaf = path->nodes[0];
+	extent = btrfs_item_ptr(leaf, path->slots[0], struct btrfs_dev_extent);
+	btrfs_set_dev_extent_chunk_tree(leaf, extent, BTRFS_CHUNK_TREE_OBJECTID);
+	btrfs_set_dev_extent_chunk_objectid(leaf, extent,
+					    BTRFS_FIRST_CHUNK_TREE_OBJECTID);
+	btrfs_set_dev_extent_chunk_offset(leaf, extent, chunk_offset);
+
+	btrfs_set_dev_extent_length(leaf, extent, num_bytes);
+	btrfs_mark_buffer_dirty(leaf);
+out:
+	btrfs_free_path(path);
+	return ret;
+}
+
+/*
+ * This function belongs to phase 2.
+ *
+ * See the comment at btrfs_chunk_alloc() for details about the chunk allocation
+ * phases.
+ */
+static int insert_dev_extents(struct btrfs_trans_handle *trans,
+				   u64 chunk_offset, u64 chunk_size)
+{
+	struct btrfs_fs_info *fs_info = trans->fs_info;
+	struct btrfs_device *device;
+	struct extent_map *em;
+	struct map_lookup *map;
+	u64 dev_offset;
+	u64 stripe_size;
+	int i;
+	int ret = 0;
+
+	em = btrfs_get_chunk_map(fs_info, chunk_offset, chunk_size);
+	if (IS_ERR(em))
+		return PTR_ERR(em);
+
+	map = em->map_lookup;
+	stripe_size = em->orig_block_len;
+
+	/*
+	 * Take the device list mutex to prevent races with the final phase of
+	 * a device replace operation that replaces the device object associated
+	 * with the map's stripes, because the device object's id can change
+	 * at any time during that final phase of the device replace operation
+	 * (dev-replace.c:btrfs_dev_replace_finishing()), so we could grab the
+	 * replaced device and then see it with an ID of BTRFS_DEV_REPLACE_DEVID,
+	 * resulting in persisting a device extent item with such ID.
+	 */
+	mutex_lock(&fs_info->fs_devices->device_list_mutex);
+	for (i = 0; i < map->num_stripes; i++) {
+		device = map->stripes[i].dev;
+		dev_offset = map->stripes[i].physical;
+
+		ret = insert_dev_extent(trans, device, chunk_offset, dev_offset,
+				       stripe_size);
+		if (ret)
+			break;
+	}
+	mutex_unlock(&fs_info->fs_devices->device_list_mutex);
+
+	free_extent_map(em);
+	return ret;
+}
+
+/*
+ * This function, btrfs_create_pending_block_groups(), belongs to the phase 2 of
+ * chunk allocation.
+ *
+ * See the comment at btrfs_chunk_alloc() for details about the chunk allocation
+ * phases.
+ */
 void btrfs_create_pending_block_groups(struct btrfs_trans_handle *trans)
 {
 	struct btrfs_fs_info *fs_info = trans->fs_info;
 	struct btrfs_block_group *block_group;
 	int ret = 0;
 
-	if (!trans->can_flush_pending_bgs)
-		return;
-
 	while (!list_empty(&trans->new_bgs)) {
 		int index;
 
@@ -2242,8 +2379,15 @@ void btrfs_create_pending_block_groups(struct btrfs_trans_handle *trans)
 		ret = insert_block_group_item(trans, block_group);
 		if (ret)
 			btrfs_abort_transaction(trans, ret);
-		ret = btrfs_finish_chunk_alloc(trans, block_group->start,
-					block_group->length);
+		if (!block_group->chunk_item_inserted) {
+			mutex_lock(&fs_info->chunk_mutex);
+			ret = btrfs_chunk_alloc_add_chunk_item(trans, block_group);
+			mutex_unlock(&fs_info->chunk_mutex);
+			if (ret)
+				btrfs_abort_transaction(trans, ret);
+		}
+		ret = insert_dev_extents(trans, block_group->start,
+					 block_group->length);
 		if (ret)
 			btrfs_abort_transaction(trans, ret);
 		add_block_group_free_space(trans, block_group);
@@ -2265,8 +2409,9 @@ next:
 	btrfs_trans_release_chunk_metadata(trans);
 }
 
-int btrfs_make_block_group(struct btrfs_trans_handle *trans, u64 bytes_used,
-			   u64 type, u64 chunk_offset, u64 size)
+struct btrfs_block_group *btrfs_make_block_group(struct btrfs_trans_handle *trans,
+						 u64 bytes_used, u64 type,
+						 u64 chunk_offset, u64 size)
 {
 	struct btrfs_fs_info *fs_info = trans->fs_info;
 	struct btrfs_block_group *cache;
@@ -2276,7 +2421,7 @@ int btrfs_make_block_group(struct btrfs_trans_handle *trans, u64 bytes_used,
 
 	cache = btrfs_create_block_group_cache(fs_info, chunk_offset);
 	if (!cache)
-		return -ENOMEM;
+		return ERR_PTR(-ENOMEM);
 
 	cache->length = size;
 	set_free_space_tree_thresholds(cache);
@@ -2290,7 +2435,7 @@ int btrfs_make_block_group(struct btrfs_trans_handle *trans, u64 bytes_used,
 	ret = btrfs_load_block_group_zone_info(cache, true);
 	if (ret) {
 		btrfs_put_block_group(cache);
-		return ret;
+		return ERR_PTR(ret);
 	}
 
 	ret = exclude_super_stripes(cache);
@@ -2298,7 +2443,7 @@ int btrfs_make_block_group(struct btrfs_trans_handle *trans, u64 bytes_used,
 		/* We may have excluded something, so call this just in case */
 		btrfs_free_excluded_extents(cache);
 		btrfs_put_block_group(cache);
-		return ret;
+		return ERR_PTR(ret);
 	}
 
 	add_new_free_space(cache, chunk_offset, chunk_offset + size);
@@ -2325,7 +2470,7 @@ int btrfs_make_block_group(struct btrfs_trans_handle *trans, u64 bytes_used,
 	if (ret) {
 		btrfs_remove_free_space_cache(cache);
 		btrfs_put_block_group(cache);
-		return ret;
+		return ERR_PTR(ret);
 	}
 
 	/*
@@ -2344,7 +2489,7 @@ int btrfs_make_block_group(struct btrfs_trans_handle *trans, u64 bytes_used,
 	btrfs_update_delayed_refs_rsv(trans);
 
 	set_avail_alloc_bits(fs_info, type);
-	return 0;
+	return cache;
 }
 
 /*
@@ -3222,11 +3367,203 @@ int btrfs_force_chunk_alloc(struct btrfs_trans_handle *trans, u64 type)
 	return btrfs_chunk_alloc(trans, alloc_flags, CHUNK_ALLOC_FORCE);
 }
 
+static int do_chunk_alloc(struct btrfs_trans_handle *trans, u64 flags)
+{
+	struct btrfs_block_group *bg;
+	int ret;
+
+	/*
+	 * Check if we have enough space in the system space info because we
+	 * will need to update device items in the chunk btree and insert a new
+	 * chunk item in the chunk btree as well. This will allocate a new
+	 * system block group if needed.
+	 */
+	check_system_chunk(trans, flags);
+
+	bg = btrfs_alloc_chunk(trans, flags);
+	if (IS_ERR(bg)) {
+		ret = PTR_ERR(bg);
+		goto out;
+	}
+
+	/*
+	 * If this is a system chunk allocation then stop right here and do not
+	 * add the chunk item to the chunk btree. This is to prevent a deadlock
+	 * because this system chunk allocation can be triggered while COWing
+	 * some extent buffer of the chunk btree and while holding a lock on a
+	 * parent extent buffer, in which case attempting to insert the chunk
+	 * item (or update the device item) would result in a deadlock on that
+	 * parent extent buffer. In this case defer the chunk btree updates to
+	 * the second phase of chunk allocation and keep our reservation until
+	 * the second phase completes.
+	 *
+	 * This is a rare case and can only be triggered by the very few cases
+	 * we have where we need to touch the chunk btree outside chunk allocation
+	 * and chunk removal. These cases are basically adding a device, removing
+	 * a device or resizing a device.
+	 */
+	if (flags & BTRFS_BLOCK_GROUP_SYSTEM)
+		return 0;
+
+	ret = btrfs_chunk_alloc_add_chunk_item(trans, bg);
+	/*
+	 * Normally we are not expected to fail with -ENOSPC here, since we have
+	 * previously reserved space in the system space_info and allocated one
+	 * new system chunk if necessary. However there are two exceptions:
+	 *
+	 * 1) We may have enough free space in the system space_info but all the
+	 *    existing system block groups have a profile which can not be used
+	 *    for extent allocation.
+	 *
+	 *    This happens when mounting in degraded mode. For example we have a
+	 *    RAID1 filesystem with 2 devices, lose one device and mount the fs
+	 *    using the other device in degraded mode. If we then allocate a chunk,
+	 *    we may have enough free space in the existing system space_info, but
+	 *    none of the block groups can be used for extent allocation since they
+	 *    have a RAID1 profile, and because we are in degraded mode with a
+	 *    single device, we are forced to allocate a new system chunk with a
+	 *    SINGLE profile. Making check_system_chunk() iterate over all system
+	 *    block groups and check if they have a usable profile and enough space
+	 *    can be slow on very large filesystems, so we tolerate the -ENOSPC and
+	 *    try again after forcing allocation of a new system chunk. Like this
+	 *    we avoid paying the cost of that search in normal circumstances, when
+	 *    we were not mounted in degraded mode;
+	 *
+	 * 2) We had enough free space info the system space_info, and one suitable
+	 *    block group to allocate from when we called check_system_chunk()
+	 *    above. However right after we called it, the only system block group
+	 *    with enough free space got turned into RO mode by a running scrub,
+	 *    and in this case we have to allocate a new one and retry. We only
+	 *    need do this allocate and retry once, since we have a transaction
+	 *    handle and scrub uses the commit root to search for block groups.
+	 */
+	if (ret == -ENOSPC) {
+		const u64 sys_flags = btrfs_system_alloc_profile(trans->fs_info);
+		struct btrfs_block_group *sys_bg;
+
+		sys_bg = btrfs_alloc_chunk(trans, sys_flags);
+		if (IS_ERR(sys_bg)) {
+			ret = PTR_ERR(sys_bg);
+			btrfs_abort_transaction(trans, ret);
+			goto out;
+		}
+
+		ret = btrfs_chunk_alloc_add_chunk_item(trans, sys_bg);
+		if (ret) {
+			btrfs_abort_transaction(trans, ret);
+			goto out;
+		}
+
+		ret = btrfs_chunk_alloc_add_chunk_item(trans, bg);
+		if (ret) {
+			btrfs_abort_transaction(trans, ret);
+			goto out;
+		}
+	} else if (ret) {
+		btrfs_abort_transaction(trans, ret);
+		goto out;
+	}
+out:
+	btrfs_trans_release_chunk_metadata(trans);
+
+	return ret;
+}
+
 /*
- * If force is CHUNK_ALLOC_FORCE:
+ * Chunk allocation is done in 2 phases:
+ *
+ * 1) Phase 1 - through btrfs_chunk_alloc() we allocate device extents for
+ *    the chunk, the chunk mapping, create its block group and add the items
+ *    that belong in the chunk btree to it - more specifically, we need to
+ *    update device items in the chunk btree and add a new chunk item to it.
+ *
+ * 2) Phase 2 - through btrfs_create_pending_block_groups(), we add the block
+ *    group item to the extent btree and the device extent items to the devices
+ *    btree.
+ *
+ * This is done to prevent deadlocks. For example when COWing a node from the
+ * extent btree we are holding a write lock on the node's parent and if we
+ * trigger chunk allocation and attempted to insert the new block group item
+ * in the extent btree right way, we could deadlock because the path for the
+ * insertion can include that parent node. At first glance it seems impossible
+ * to trigger chunk allocation after starting a transaction since tasks should
+ * reserve enough transaction units (metadata space), however while that is true
+ * most of the time, chunk allocation may still be triggered for several reasons:
+ *
+ * 1) When reserving metadata, we check if there is enough free space in the
+ *    metadata space_info and therefore don't trigger allocation of a new chunk.
+ *    However later when the task actually tries to COW an extent buffer from
+ *    the extent btree or from the device btree for example, it is forced to
+ *    allocate a new block group (chunk) because the only one that had enough
+ *    free space was just turned to RO mode by a running scrub for example (or
+ *    device replace, block group reclaim thread, etc), so we can not use it
+ *    for allocating an extent and end up being forced to allocate a new one;
+ *
+ * 2) Because we only check that the metadata space_info has enough free bytes,
+ *    we end up not allocating a new metadata chunk in that case. However if
+ *    the filesystem was mounted in degraded mode, none of the existing block
+ *    groups might be suitable for extent allocation due to their incompatible
+ *    profile (for e.g. mounting a 2 devices filesystem, where all block groups
+ *    use a RAID1 profile, in degraded mode using a single device). In this case
+ *    when the task attempts to COW some extent buffer of the extent btree for
+ *    example, it will trigger allocation of a new metadata block group with a
+ *    suitable profile (SINGLE profile in the example of the degraded mount of
+ *    the RAID1 filesystem);
+ *
+ * 3) The task has reserved enough transaction units / metadata space, but when
+ *    it attempts to COW an extent buffer from the extent or device btree for
+ *    example, it does not find any free extent in any metadata block group,
+ *    therefore forced to try to allocate a new metadata block group.
+ *    This is because some other task allocated all available extents in the
+ *    meanwhile - this typically happens with tasks that don't reserve space
+ *    properly, either intentionally or as a bug. One example where this is
+ *    done intentionally is fsync, as it does not reserve any transaction units
+ *    and ends up allocating a variable number of metadata extents for log
+ *    tree extent buffers.
+ *
+ * We also need this 2 phases setup when adding a device to a filesystem with
+ * a seed device - we must create new metadata and system chunks without adding
+ * any of the block group items to the chunk, extent and device btrees. If we
+ * did not do it this way, we would get ENOSPC when attempting to update those
+ * btrees, since all the chunks from the seed device are read-only.
+ *
+ * Phase 1 does the updates and insertions to the chunk btree because if we had
+ * it done in phase 2 and have a thundering herd of tasks allocating chunks in
+ * parallel, we risk having too many system chunks allocated by many tasks if
+ * many tasks reach phase 1 without the previous ones completing phase 2. In the
+ * extreme case this leads to exhaustion of the system chunk array in the
+ * superblock. This is easier to trigger if using a btree node/leaf size of 64K
+ * and with RAID filesystems (so we have more device items in the chunk btree).
+ * This has happened before and commit eafa4fd0ad0607 ("btrfs: fix exhaustion of
+ * the system chunk array due to concurrent allocations") provides more details.
+ *
+ * For allocation of system chunks, we defer the updates and insertions into the
+ * chunk btree to phase 2. This is to prevent deadlocks on extent buffers because
+ * if the chunk allocation is triggered while COWing an extent buffer of the
+ * chunk btree, we are holding a lock on the parent of that extent buffer and
+ * doing the chunk btree updates and insertions can require locking that parent.
+ * This is for the very few and rare cases where we update the chunk btree that
+ * are not chunk allocation or chunk removal: adding a device, removing a device
+ * or resizing a device.
+ *
+ * The reservation of system space, done through check_system_chunk(), as well
+ * as all the updates and insertions into the chunk btree must be done while
+ * holding fs_info->chunk_mutex. This is important to guarantee that while COWing
+ * an extent buffer from the chunks btree we never trigger allocation of a new
+ * system chunk, which would result in a deadlock (trying to lock twice an
+ * extent buffer of the chunk btree, first time before triggering the chunk
+ * allocation and the second time during chunk allocation while attempting to
+ * update the chunks btree). The system chunk array is also updated while holding
+ * that mutex. The same logic applies to removing chunks - we must reserve system
+ * space, update the chunk btree and the system chunk array in the superblock
+ * while holding fs_info->chunk_mutex.
+ *
+ * This function, btrfs_chunk_alloc(), belongs to phase 1.
+ *
+ * If @force is CHUNK_ALLOC_FORCE:
  *    - return 1 if it successfully allocates a chunk,
  *    - return errors including -ENOSPC otherwise.
- * If force is NOT CHUNK_ALLOC_FORCE:
+ * If @force is NOT CHUNK_ALLOC_FORCE:
  *    - return 0 if it doesn't need to allocate a new chunk,
  *    - return 1 if it successfully allocates a chunk,
  *    - return errors including -ENOSPC otherwise.
@@ -3243,6 +3580,13 @@ int btrfs_chunk_alloc(struct btrfs_trans_handle *trans, u64 flags,
 	/* Don't re-enter if we're already allocating a chunk */
 	if (trans->allocating_chunk)
 		return -ENOSPC;
+	/*
+	 * If we are removing a chunk, don't re-enter or we would deadlock.
+	 * System space reservation and system chunk allocation is done by the
+	 * chunk remove operation (btrfs_remove_chunk()).
+	 */
+	if (trans->removing_chunk)
+		return -ENOSPC;
 
 	space_info = btrfs_find_space_info(fs_info, flags);
 	ASSERT(space_info);
@@ -3306,13 +3650,7 @@ int btrfs_chunk_alloc(struct btrfs_trans_handle *trans, u64 flags,
 			force_metadata_allocation(fs_info);
 	}
 
-	/*
-	 * Check if we have enough space in SYSTEM chunk because we may need
-	 * to update devices.
-	 */
-	check_system_chunk(trans, flags);
-
-	ret = btrfs_alloc_chunk(trans, flags);
+	ret = do_chunk_alloc(trans, flags);
 	trans->allocating_chunk = false;
 
 	spin_lock(&space_info->lock);
@@ -3331,22 +3669,6 @@ out:
 	space_info->chunk_alloc = 0;
 	spin_unlock(&space_info->lock);
 	mutex_unlock(&fs_info->chunk_mutex);
-	/*
-	 * When we allocate a new chunk we reserve space in the chunk block
-	 * reserve to make sure we can COW nodes/leafs in the chunk tree or
-	 * add new nodes/leafs to it if we end up needing to do it when
-	 * inserting the chunk item and updating device items as part of the
-	 * second phase of chunk allocation, performed by
-	 * btrfs_finish_chunk_alloc(). So make sure we don't accumulate a
-	 * large number of new block groups to create in our transaction
-	 * handle's new_bgs list to avoid exhausting the chunk block reserve
-	 * in extreme cases - like having a single transaction create many new
-	 * block groups when starting to write out the free space caches of all
-	 * the block groups that were made dirty during the lifetime of the
-	 * transaction.
-	 */
-	if (trans->chunk_bytes_reserved >= (u64)SZ_2M)
-		btrfs_create_pending_block_groups(trans);
 
 	return ret;
 }
@@ -3367,7 +3689,6 @@ static u64 get_profile_num_devs(struct btrfs_fs_info *fs_info, u64 type)
  */
 void check_system_chunk(struct btrfs_trans_handle *trans, u64 type)
 {
-	struct btrfs_transaction *cur_trans = trans->transaction;
 	struct btrfs_fs_info *fs_info = trans->fs_info;
 	struct btrfs_space_info *info;
 	u64 left;
@@ -3382,7 +3703,6 @@ void check_system_chunk(struct btrfs_trans_handle *trans, u64 type)
 	lockdep_assert_held(&fs_info->chunk_mutex);
 
 	info = btrfs_find_space_info(fs_info, BTRFS_BLOCK_GROUP_SYSTEM);
-again:
 	spin_lock(&info->lock);
 	left = info->total_bytes - btrfs_space_info_used(info, true);
 	spin_unlock(&info->lock);
@@ -3401,76 +3721,39 @@ again:
 
 	if (left < thresh) {
 		u64 flags = btrfs_system_alloc_profile(fs_info);
-		u64 reserved = atomic64_read(&cur_trans->chunk_bytes_reserved);
-
-		/*
-		 * If there's not available space for the chunk tree (system
-		 * space) and there are other tasks that reserved space for
-		 * creating a new system block group, wait for them to complete
-		 * the creation of their system block group and release excess
-		 * reserved space. We do this because:
-		 *
-		 * *) We can end up allocating more system chunks than necessary
-		 *    when there are multiple tasks that are concurrently
-		 *    allocating block groups, which can lead to exhaustion of
-		 *    the system array in the superblock;
-		 *
-		 * *) If we allocate extra and unnecessary system block groups,
-		 *    despite being empty for a long time, and possibly forever,
-		 *    they end not being added to the list of unused block groups
-		 *    because that typically happens only when deallocating the
-		 *    last extent from a block group - which never happens since
-		 *    we never allocate from them in the first place. The few
-		 *    exceptions are when mounting a filesystem or running scrub,
-		 *    which add unused block groups to the list of unused block
-		 *    groups, to be deleted by the cleaner kthread.
-		 *    And even when they are added to the list of unused block
-		 *    groups, it can take a long time until they get deleted,
-		 *    since the cleaner kthread might be sleeping or busy with
-		 *    other work (deleting subvolumes, running delayed iputs,
-		 *    defrag scheduling, etc);
-		 *
-		 * This is rare in practice, but can happen when too many tasks
-		 * are allocating blocks groups in parallel (via fallocate())
-		 * and before the one that reserved space for a new system block
-		 * group finishes the block group creation and releases the space
-		 * reserved in excess (at btrfs_create_pending_block_groups()),
-		 * other tasks end up here and see free system space temporarily
-		 * not enough for updating the chunk tree.
-		 *
-		 * We unlock the chunk mutex before waiting for such tasks and
-		 * lock it again after the wait, otherwise we would deadlock.
-		 * It is safe to do so because allocating a system chunk is the
-		 * first thing done while allocating a new block group.
-		 */
-		if (reserved > trans->chunk_bytes_reserved) {
-			const u64 min_needed = reserved - thresh;
-
-			mutex_unlock(&fs_info->chunk_mutex);
-			wait_event(cur_trans->chunk_reserve_wait,
-			   atomic64_read(&cur_trans->chunk_bytes_reserved) <=
-			   min_needed);
-			mutex_lock(&fs_info->chunk_mutex);
-			goto again;
-		}
+		struct btrfs_block_group *bg;
 
 		/*
 		 * Ignore failure to create system chunk. We might end up not
 		 * needing it, as we might not need to COW all nodes/leafs from
 		 * the paths we visit in the chunk tree (they were already COWed
 		 * or created in the current transaction for example).
+		 *
+		 * Also, if our caller is allocating a system chunk, do not
+		 * attempt to insert the chunk item in the chunk btree, as we
+		 * could deadlock on an extent buffer since our caller may be
+		 * COWing an extent buffer from the chunk btree.
 		 */
-		ret = btrfs_alloc_chunk(trans, flags);
+		bg = btrfs_alloc_chunk(trans, flags);
+		if (IS_ERR(bg)) {
+			ret = PTR_ERR(bg);
+		} else if (!(type & BTRFS_BLOCK_GROUP_SYSTEM)) {
+			/*
+			 * If we fail to add the chunk item here, we end up
+			 * trying again at phase 2 of chunk allocation, at
+			 * btrfs_create_pending_block_groups(). So ignore
+			 * any error here.
+			 */
+			btrfs_chunk_alloc_add_chunk_item(trans, bg);
+		}
 	}
 
 	if (!ret) {
 		ret = btrfs_block_rsv_add(fs_info->chunk_root,
 					  &fs_info->chunk_block_rsv,
 					  thresh, BTRFS_RESERVE_NO_FLUSH);
-		if (!ret) {
-			atomic64_add(thresh, &cur_trans->chunk_bytes_reserved);
+		if (!ret)
 			trans->chunk_bytes_reserved += thresh;
-		}
 	}
 }