btrfs: remove btrfs_root's delalloc_mutex

When running metadata space reclaim under high I/O concurrency, we observe
hung tasks caused by lock contention on `btrfs_root::delalloc_mutex`. For
example:

  INFO: task kworker/u132:1:2177 blocked for more than 122 seconds.
        Not tainted 6.16.0-rc3+ torvalds#1246
  Workqueue: events_unbound btrfs_preempt_reclaim_metadata_space
  Call Trace:
    __schedule+0x2f9/0x7b0
    schedule+0x27/0x80
    __mutex_lock.constprop.0+0x4af/0x890
    start_delalloc_inodes+0x6e/0x400
    btrfs_start_delalloc_roots+0x162/0x270
    shrink_delalloc+0x10c/0x2d0
    flush_space+0x202/0x280
    btrfs_preempt_reclaim_metadata_space+0xe7/0x340

The `delalloc_mutex` serializes delalloc flushing per root but is no
longer necessary. All critical paths (inode flushing, extent writing,
metadata updates) are already synchronized using finer-grained locking at
the inode, page, and tree levels. In particular, concurrent flushers
coordinate via inode locking, and no shared state requires global
serialization across the root.

Removing this mutex avoids unnecessary blocking in reclaim paths and
improves responsiveness under pressure, especially on systems with many
flushers or multi-queue SSDs/ZNS devices.

Signed-off-by: Johannes Thumshirn <[email protected]>

Signed-off-by: Johannes Thumshirn <[email protected]>

Author: morbidrsa

fs/btrfs/ctree.h

@@ -238,7 +238,6 @@ struct btrfs_root {
 	spinlock_t root_item_lock;
 	refcount_t refs;
 
-	struct mutex delalloc_mutex;
 	spinlock_t delalloc_lock;
 	/*
 	 * all of the inodes that have delalloc bytes.  It is possible for

fs/btrfs/disk-io.c

@@ -678,7 +678,6 @@ static struct btrfs_root *btrfs_alloc_root(struct btrfs_fs_info *fs_info,
 	mutex_init(&root->objectid_mutex);
 	mutex_init(&root->log_mutex);
 	mutex_init(&root->ordered_extent_mutex);
-	mutex_init(&root->delalloc_mutex);
 	init_waitqueue_head(&root->qgroup_flush_wait);
 	init_waitqueue_head(&root->log_writer_wait);
 	init_waitqueue_head(&root->log_commit_wait[0]);

fs/btrfs/inode.c

@@ -8673,7 +8673,6 @@ static int start_delalloc_inodes(struct btrfs_root *root,
 	int ret = 0;
 	bool full_flush = wbc->nr_to_write == LONG_MAX;
 
-	mutex_lock(&root->delalloc_mutex);
 	spin_lock(&root->delalloc_lock);
 	list_splice_init(&root->delalloc_inodes, &splice);
 	while (!list_empty(&splice)) {
@@ -8730,7 +8729,6 @@ static int start_delalloc_inodes(struct btrfs_root *root,
 		list_splice_tail(&splice, &root->delalloc_inodes);
 		spin_unlock(&root->delalloc_lock);
 	}
-	mutex_unlock(&root->delalloc_mutex);
 	return ret;
 }