btrfs: obsolete and remove mount option alloc_start

The mount option alloc_start was used in the past for debugging and
stressing the chunk allocator. Not meant to be used by users, so we're
not breaking anybody's setup.

There was some added complexity handling changes of the value and when
it was not same as default. Such code has likely been untested and I
think it's better to remove it.

This patch kills all use of alloc_start, and by doing that also fixes
a bug when alloc_size is set, potentially called from statfs:

in btrfs_calc_avail_data_space, traversing the list in RCU, the RCU
protection is temporarily dropped so btrfs_account_dev_extents_size can
be called and then RCU is locked again! Doing that inside
list_for_each_entry_rcu is just asking for trouble, but unlikely to be
observed in practice.

Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 1 insertions, 11 deletions

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 6375e57a5a69..d868509afd5b 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -799,17 +799,7 @@ struct btrfs_fs_info {
 	 * so it is also safe.
 	 */
 	u64 max_inline;
-	/*
-	 * Protected by ->chunk_mutex and sb->s_umount.
-	 *
-	 * The reason that we use two lock to protect it is because only
-	 * remount and mount operations can change it and these two operations
-	 * are under sb->s_umount, but the read side (chunk allocation) can not
-	 * acquire sb->s_umount or the deadlock would happen. So we use two
-	 * locks to protect it. On the write side, we must acquire two locks,
-	 * and on the read side, we just need acquire one of them.
-	 */
-	u64 alloc_start;
+
 	struct btrfs_transaction *running_transaction;
 	wait_queue_head_t transaction_throttle;
 	wait_queue_head_t transaction_wait;