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btrfs: set blocking_writers directly, no increment or decrement 

The increment and decrement was inherited from previous version that
used atomics, switched in commit 06297d8cef ("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 06297d8cef):

     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>
This commit is contained in:
David Sterba 2019-10-10 23:31:19 +02:00
parent f5c2a52590
commit 40d38f53d4
1 changed files with 2 additions and 2 deletions
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4
fs/btrfs/locking.c
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@ -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