[PATCH v2 2/5] btrfs: set blocking_writers directly, no increment or decrement

David Sterba
2019-10-30T10:56:52Z

The increment and decrement was inherited from previous version that
used atomics, switched in commit 06297d8cefca ("btrfs: switch
extent_buffer blocking_writers from atomic to int"). The only possible
values are 0 and 1 so we can set them directly.

The generated assembly (gcc 9.x) did the direct value assignment in
btrfs_set_lock_blocking_write (asm diff after change in 06297d8cefca):

5d: test %eax,%eax
5f: je 62 <btrfs_set_lock_blocking_write+0x22>
61: retq

- 62: lock incl 0x44(%rdi)
- 66: add $0x50,%rdi
- 6a: jmpq 6f <btrfs_set_lock_blocking_write+0x2f>

+ 62: movl $0x1,0x44(%rdi)
+ 69: add $0x50,%rdi
+ 6d: jmpq 72 <btrfs_set_lock_blocking_write+0x32>

The part in btrfs_tree_unlock did a decrement because
BUG_ON(blockers > 1) is probably not a strong hint for the compiler, but
otherwise the output looks safe:

- lock decl 0x44(%rdi)

+ sub $0x1,%eax
+ mov %eax,0x44(%rdi)

Signed-off-by: David Sterba <dsterba@suse.com>
---
fs/btrfs/locking.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/locking.c b/fs/btrfs/locking.c
index c84c650e56c7..00edf91c3d1c 100644
--- a/fs/btrfs/locking.c
+++ b/fs/btrfs/locking.c
@@ -109,7 +109,7 @@ void btrfs_set_lock_blocking_write(struct extent_buffer *eb)
if (eb->blocking_writers == 0) {
btrfs_assert_spinning_writers_put(eb);
btrfs_assert_tree_locked(eb);
- eb->blocking_writers++;
+ eb->blocking_writers = 1;
write_unlock(&eb->lock);
}
}
@@ -305,7 +305,7 @@ void btrfs_tree_unlock(struct extent_buffer *eb)

if (blockers) {
btrfs_assert_no_spinning_writers(eb);
- eb->blocking_writers--;
+ eb->blocking_writers = 0;
/*
* We need to order modifying blocking_writers above with
* actually waking up the sleepers to ensure they see the
--
2.23.0