The patch below does not apply to the 5.14-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
From 2bb2e00ed9787e52580bb651264b8d6a2b7a9dd2 Mon Sep 17 00:00:00 2001
From: Filipe Manana fdmanana@suse.com Date: Wed, 13 Oct 2021 10:12:49 +0100 Subject: [PATCH] btrfs: fix deadlock between chunk allocation and chunk btree modifications
When a task is doing some modification to the chunk btree and it is not in the context of a chunk allocation or a chunk removal, it can deadlock with another task that is currently allocating a new data or metadata chunk.
These contexts are the following:
* When relocating a system chunk, when we need to COW the extent buffers that belong to the chunk btree;
* When adding a new device (ioctl), where we need to add a new device item to the chunk btree;
* When removing a device (ioctl), where we need to remove a device item from the chunk btree;
* When resizing a device (ioctl), where we need to update a device item in the chunk btree and may need to relocate a system chunk that lies beyond the new device size when shrinking a device.
The problem happens due to a sequence of steps like the following:
1) Task A starts a data or metadata chunk allocation and it locks the chunk mutex;
2) Task B is relocating a system chunk, and when it needs to COW an extent buffer of the chunk btree, it has locked both that extent buffer as well as its parent extent buffer;
3) Since there is not enough available system space, either because none of the existing system block groups have enough free space or because the only one with enough free space is in RO mode due to the relocation, task B triggers a new system chunk allocation. It blocks when trying to acquire the chunk mutex, currently held by task A;
4) Task A enters btrfs_chunk_alloc_add_chunk_item(), in order to insert the new chunk item into the chunk btree and update the existing device items there. But in order to do that, it has to lock the extent buffer that task B locked at step 2, or its parent extent buffer, but task B is waiting on the chunk mutex, which is currently locked by task A, therefore resulting in a deadlock.
One example report when the deadlock happens with system chunk relocation:
INFO: task kworker/u9:5:546 blocked for more than 143 seconds. Not tainted 5.15.0-rc3+ #1 "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. task:kworker/u9:5 state:D stack:25936 pid: 546 ppid: 2 flags:0x00004000 Workqueue: events_unbound btrfs_async_reclaim_metadata_space Call Trace: context_switch kernel/sched/core.c:4940 [inline] __schedule+0xcd9/0x2530 kernel/sched/core.c:6287 schedule+0xd3/0x270 kernel/sched/core.c:6366 rwsem_down_read_slowpath+0x4ee/0x9d0 kernel/locking/rwsem.c:993 __down_read_common kernel/locking/rwsem.c:1214 [inline] __down_read kernel/locking/rwsem.c:1223 [inline] down_read_nested+0xe6/0x440 kernel/locking/rwsem.c:1590 __btrfs_tree_read_lock+0x31/0x350 fs/btrfs/locking.c:47 btrfs_tree_read_lock fs/btrfs/locking.c:54 [inline] btrfs_read_lock_root_node+0x8a/0x320 fs/btrfs/locking.c:191 btrfs_search_slot_get_root fs/btrfs/ctree.c:1623 [inline] btrfs_search_slot+0x13b4/0x2140 fs/btrfs/ctree.c:1728 btrfs_update_device+0x11f/0x500 fs/btrfs/volumes.c:2794 btrfs_chunk_alloc_add_chunk_item+0x34d/0xea0 fs/btrfs/volumes.c:5504 do_chunk_alloc fs/btrfs/block-group.c:3408 [inline] btrfs_chunk_alloc+0x84d/0xf50 fs/btrfs/block-group.c:3653 flush_space+0x54e/0xd80 fs/btrfs/space-info.c:670 btrfs_async_reclaim_metadata_space+0x396/0xa90 fs/btrfs/space-info.c:953 process_one_work+0x9df/0x16d0 kernel/workqueue.c:2297 worker_thread+0x90/0xed0 kernel/workqueue.c:2444 kthread+0x3e5/0x4d0 kernel/kthread.c:319 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295 INFO: task syz-executor:9107 blocked for more than 143 seconds. Not tainted 5.15.0-rc3+ #1 "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. task:syz-executor state:D stack:23200 pid: 9107 ppid: 7792 flags:0x00004004 Call Trace: context_switch kernel/sched/core.c:4940 [inline] __schedule+0xcd9/0x2530 kernel/sched/core.c:6287 schedule+0xd3/0x270 kernel/sched/core.c:6366 schedule_preempt_disabled+0xf/0x20 kernel/sched/core.c:6425 __mutex_lock_common kernel/locking/mutex.c:669 [inline] __mutex_lock+0xc96/0x1680 kernel/locking/mutex.c:729 btrfs_chunk_alloc+0x31a/0xf50 fs/btrfs/block-group.c:3631 find_free_extent_update_loop fs/btrfs/extent-tree.c:3986 [inline] find_free_extent+0x25cb/0x3a30 fs/btrfs/extent-tree.c:4335 btrfs_reserve_extent+0x1f1/0x500 fs/btrfs/extent-tree.c:4415 btrfs_alloc_tree_block+0x203/0x1120 fs/btrfs/extent-tree.c:4813 __btrfs_cow_block+0x412/0x1620 fs/btrfs/ctree.c:415 btrfs_cow_block+0x2f6/0x8c0 fs/btrfs/ctree.c:570 btrfs_search_slot+0x1094/0x2140 fs/btrfs/ctree.c:1768 relocate_tree_block fs/btrfs/relocation.c:2694 [inline] relocate_tree_blocks+0xf73/0x1770 fs/btrfs/relocation.c:2757 relocate_block_group+0x47e/0xc70 fs/btrfs/relocation.c:3673 btrfs_relocate_block_group+0x48a/0xc60 fs/btrfs/relocation.c:4070 btrfs_relocate_chunk+0x96/0x280 fs/btrfs/volumes.c:3181 __btrfs_balance fs/btrfs/volumes.c:3911 [inline] btrfs_balance+0x1f03/0x3cd0 fs/btrfs/volumes.c:4301 btrfs_ioctl_balance+0x61e/0x800 fs/btrfs/ioctl.c:4137 btrfs_ioctl+0x39ea/0x7b70 fs/btrfs/ioctl.c:4949 vfs_ioctl fs/ioctl.c:51 [inline] __do_sys_ioctl fs/ioctl.c:874 [inline] __se_sys_ioctl fs/ioctl.c:860 [inline] __x64_sys_ioctl+0x193/0x200 fs/ioctl.c:860 do_syscall_x64 arch/x86/entry/common.c:50 [inline] do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80 entry_SYSCALL_64_after_hwframe+0x44/0xae
So fix this by making sure that whenever we try to modify the chunk btree and we are neither in a chunk allocation context nor in a chunk remove context, we reserve system space before modifying the chunk btree.
Reported-by: Hao Sun sunhao.th@gmail.com Link: https://lore.kernel.org/linux-btrfs/CACkBjsax51i4mu6C0C3vJqQN3NR_iVuucoeG3U1... Fixes: 79bd37120b1495 ("btrfs: rework chunk allocation to avoid exhaustion of the system chunk array") CC: stable@vger.kernel.org # 5.14+ Reviewed-by: Josef Bacik josef@toxicpanda.com Signed-off-by: Filipe Manana fdmanana@suse.com Signed-off-by: David Sterba dsterba@suse.com
diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c index de9aeb3733cf..f971d043469c 100644 --- a/fs/btrfs/block-group.c +++ b/fs/btrfs/block-group.c @@ -3425,25 +3425,6 @@ static int do_chunk_alloc(struct btrfs_trans_handle *trans, u64 flags) 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 @@ -3576,14 +3557,14 @@ out: * 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. + * Allocation of system chunks does not happen through this function. A task that + * needs to update the chunk btree (the only btree that uses system chunks), must + * preallocate chunk space by calling either check_system_chunk() or + * btrfs_reserve_chunk_metadata() - the former is used when allocating a data or + * metadata chunk or when removing a chunk, while the later is used before doing + * a modification to the chunk btree - use cases for the later are adding, + * removing and resizing a device as well as relocation of a system chunk. + * See the comment below for more details. * * 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 @@ -3620,11 +3601,27 @@ int btrfs_chunk_alloc(struct btrfs_trans_handle *trans, u64 flags, 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()). + * Allocation of system chunks can not happen through this path, as we + * could end up in a deadlock if we are allocating a data or metadata + * chunk and there is another task modifying the chunk btree. + * + * This is because while we are holding the chunk mutex, we will attempt + * to add the new chunk item to the chunk btree or update an existing + * device item in the chunk btree, while the other task that is modifying + * the chunk btree is attempting to COW an extent buffer while holding a + * lock on it and on its parent - if the COW operation triggers a system + * chunk allocation, then we can deadlock because we are holding the + * chunk mutex and we may need to access that extent buffer or its parent + * in order to add the chunk item or update a device item. + * + * Tasks that want to modify the chunk tree should reserve system space + * before updating the chunk btree, by calling either + * btrfs_reserve_chunk_metadata() or check_system_chunk(). + * It's possible that after a task reserves the space, it still ends up + * here - this happens in the cases described above at do_chunk_alloc(). + * The task will have to either retry or fail. */ - if (trans->removing_chunk) + if (flags & BTRFS_BLOCK_GROUP_SYSTEM) return -ENOSPC;
space_info = btrfs_find_space_info(fs_info, flags); @@ -3723,17 +3720,14 @@ static u64 get_profile_num_devs(struct btrfs_fs_info *fs_info, u64 type) return num_dev; }
-/* - * Reserve space in the system space for allocating or removing a chunk - */ -void check_system_chunk(struct btrfs_trans_handle *trans, u64 type) +static void reserve_chunk_space(struct btrfs_trans_handle *trans, + u64 bytes, + u64 type) { struct btrfs_fs_info *fs_info = trans->fs_info; struct btrfs_space_info *info; u64 left; - u64 thresh; int ret = 0; - u64 num_devs;
/* * Needed because we can end up allocating a system chunk and for an @@ -3746,19 +3740,13 @@ void check_system_chunk(struct btrfs_trans_handle *trans, u64 type) left = info->total_bytes - btrfs_space_info_used(info, true); spin_unlock(&info->lock);
- num_devs = get_profile_num_devs(fs_info, type); - - /* num_devs device items to update and 1 chunk item to add or remove */ - thresh = btrfs_calc_metadata_size(fs_info, num_devs) + - btrfs_calc_insert_metadata_size(fs_info, 1); - - if (left < thresh && btrfs_test_opt(fs_info, ENOSPC_DEBUG)) { + if (left < bytes && btrfs_test_opt(fs_info, ENOSPC_DEBUG)) { btrfs_info(fs_info, "left=%llu, need=%llu, flags=%llu", - left, thresh, type); + left, bytes, type); btrfs_dump_space_info(fs_info, info, 0, 0); }
- if (left < thresh) { + if (left < bytes) { u64 flags = btrfs_system_alloc_profile(fs_info); struct btrfs_block_group *bg;
@@ -3767,21 +3755,20 @@ void check_system_chunk(struct btrfs_trans_handle *trans, u64 type) * 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. */ bg = btrfs_create_chunk(trans, flags); if (IS_ERR(bg)) { ret = PTR_ERR(bg); - } else if (!(type & BTRFS_BLOCK_GROUP_SYSTEM)) { + } else { /* * 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. + * any error here. An ENOSPC here could happen, due to + * the cases described at do_chunk_alloc() - the system + * block group we just created was just turned into RO + * mode by a scrub for example, or a running discard + * temporarily removed its free space entries, etc. */ btrfs_chunk_alloc_add_chunk_item(trans, bg); } @@ -3790,12 +3777,61 @@ void check_system_chunk(struct btrfs_trans_handle *trans, u64 type) if (!ret) { ret = btrfs_block_rsv_add(fs_info->chunk_root, &fs_info->chunk_block_rsv, - thresh, BTRFS_RESERVE_NO_FLUSH); + bytes, BTRFS_RESERVE_NO_FLUSH); if (!ret) - trans->chunk_bytes_reserved += thresh; + trans->chunk_bytes_reserved += bytes; } }
+/* + * Reserve space in the system space for allocating or removing a chunk. + * The caller must be holding fs_info->chunk_mutex. + */ +void check_system_chunk(struct btrfs_trans_handle *trans, u64 type) +{ + struct btrfs_fs_info *fs_info = trans->fs_info; + const u64 num_devs = get_profile_num_devs(fs_info, type); + u64 bytes; + + /* num_devs device items to update and 1 chunk item to add or remove. */ + bytes = btrfs_calc_metadata_size(fs_info, num_devs) + + btrfs_calc_insert_metadata_size(fs_info, 1); + + reserve_chunk_space(trans, bytes, type); +} + +/* + * Reserve space in the system space, if needed, for doing a modification to the + * chunk btree. + * + * @trans: A transaction handle. + * @is_item_insertion: Indicate if the modification is for inserting a new item + * in the chunk btree or if it's for the deletion or update + * of an existing item. + * + * This is used in a context where we need to update the chunk btree outside + * block group allocation and removal, to avoid a deadlock with a concurrent + * task that is allocating a metadata or data block group and therefore needs to + * update the chunk btree while holding the chunk mutex. After the update to the + * chunk btree is done, btrfs_trans_release_chunk_metadata() should be called. + * + */ +void btrfs_reserve_chunk_metadata(struct btrfs_trans_handle *trans, + bool is_item_insertion) +{ + struct btrfs_fs_info *fs_info = trans->fs_info; + u64 bytes; + + if (is_item_insertion) + bytes = btrfs_calc_insert_metadata_size(fs_info, 1); + else + bytes = btrfs_calc_metadata_size(fs_info, 1); + + mutex_lock(&fs_info->chunk_mutex); + reserve_chunk_space(trans, bytes, BTRFS_BLOCK_GROUP_SYSTEM); + mutex_unlock(&fs_info->chunk_mutex); +} + void btrfs_put_block_group_cache(struct btrfs_fs_info *info) { struct btrfs_block_group *block_group; diff --git a/fs/btrfs/block-group.h b/fs/btrfs/block-group.h index 07f977d3816c..5878b7ce3b78 100644 --- a/fs/btrfs/block-group.h +++ b/fs/btrfs/block-group.h @@ -293,6 +293,8 @@ int btrfs_chunk_alloc(struct btrfs_trans_handle *trans, u64 flags, enum btrfs_chunk_alloc_enum force); int btrfs_force_chunk_alloc(struct btrfs_trans_handle *trans, u64 type); void check_system_chunk(struct btrfs_trans_handle *trans, const u64 type); +void btrfs_reserve_chunk_metadata(struct btrfs_trans_handle *trans, + bool is_item_insertion); u64 btrfs_get_alloc_profile(struct btrfs_fs_info *fs_info, u64 orig_flags); void btrfs_put_block_group_cache(struct btrfs_fs_info *info); int btrfs_free_block_groups(struct btrfs_fs_info *info); diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c index fed823596248..33a0ee7ac590 100644 --- a/fs/btrfs/relocation.c +++ b/fs/btrfs/relocation.c @@ -2692,8 +2692,12 @@ static int relocate_tree_block(struct btrfs_trans_handle *trans, list_add_tail(&node->list, &rc->backref_cache.changed); } else { path->lowest_level = node->level; + if (root == root->fs_info->chunk_root) + btrfs_reserve_chunk_metadata(trans, false); ret = btrfs_search_slot(trans, root, key, path, 0, 1); btrfs_release_path(path); + if (root == root->fs_info->chunk_root) + btrfs_trans_release_chunk_metadata(trans); if (ret > 0) ret = 0; } diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c index debba6f04858..9eab8a741166 100644 --- a/fs/btrfs/volumes.c +++ b/fs/btrfs/volumes.c @@ -1847,8 +1847,10 @@ static int btrfs_add_dev_item(struct btrfs_trans_handle *trans, key.type = BTRFS_DEV_ITEM_KEY; key.offset = device->devid;
+ btrfs_reserve_chunk_metadata(trans, true); ret = btrfs_insert_empty_item(trans, trans->fs_info->chunk_root, path, &key, sizeof(*dev_item)); + btrfs_trans_release_chunk_metadata(trans); if (ret) goto out;
@@ -1921,7 +1923,9 @@ static int btrfs_rm_dev_item(struct btrfs_device *device) key.type = BTRFS_DEV_ITEM_KEY; key.offset = device->devid;
+ btrfs_reserve_chunk_metadata(trans, false); ret = btrfs_search_slot(trans, root, &key, path, -1, 1); + btrfs_trans_release_chunk_metadata(trans); if (ret) { if (ret > 0) ret = -ENOENT; @@ -2513,7 +2517,9 @@ static int btrfs_finish_sprout(struct btrfs_trans_handle *trans) key.type = BTRFS_DEV_ITEM_KEY;
while (1) { + btrfs_reserve_chunk_metadata(trans, false); ret = btrfs_search_slot(trans, root, &key, path, 0, 1); + btrfs_trans_release_chunk_metadata(trans); if (ret < 0) goto error;
@@ -2862,6 +2868,7 @@ int btrfs_grow_device(struct btrfs_trans_handle *trans, struct btrfs_super_block *super_copy = fs_info->super_copy; u64 old_total; u64 diff; + int ret;
if (!test_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state)) return -EACCES; @@ -2890,7 +2897,11 @@ int btrfs_grow_device(struct btrfs_trans_handle *trans, &trans->transaction->dev_update_list); mutex_unlock(&fs_info->chunk_mutex);
- return btrfs_update_device(trans, device); + btrfs_reserve_chunk_metadata(trans, false); + ret = btrfs_update_device(trans, device); + btrfs_trans_release_chunk_metadata(trans); + + return ret; }
static int btrfs_free_chunk(struct btrfs_trans_handle *trans, u64 chunk_offset) @@ -4925,8 +4936,10 @@ again: round_down(old_total - diff, fs_info->sectorsize)); mutex_unlock(&fs_info->chunk_mutex);
+ btrfs_reserve_chunk_metadata(trans, false); /* Now btrfs_update_device() will change the on-disk size. */ ret = btrfs_update_device(trans, device); + btrfs_trans_release_chunk_metadata(trans); if (ret < 0) { btrfs_abort_transaction(trans, ret); btrfs_end_transaction(trans);