| Message ID | 20220809163633.8255-1-ubizjak@gmail.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | fs/btrfs: Use atomic_try_cmpxchg in free_extent_buffer | expand |
On Tue, Aug 09, 2022 at 06:36:33PM +0200, Uros Bizjak wrote: > Use `atomic_try_cmpxchg(ptr, &old, new)` instead of > `atomic_cmpxchg(ptr, old, new) == old` in free_extent_buffer. This > has two benefits: > > - The x86 cmpxchg instruction returns success in the ZF flag, so this > change saves a compare after cmpxchg, as well as a related move > instruction in the front of cmpxchg. > > - atomic_try_cmpxchg implicitly assigns the *ptr value to &old when > cmpxchg fails, enabling further code simplifications. > > This patch has no functional change. > > Cc: Chris Mason <clm@fb.com> > Cc: Josef Bacik <josef@toxicpanda.com> > Cc: David Sterba <dsterba@suse.com> > Signed-off-by: Uros Bizjak <ubizjak@gmail.com> Reviewed-by: Boris Burkov <boris@bur.io> > --- > fs/btrfs/extent_io.c | 6 ++---- > 1 file changed, 2 insertions(+), 4 deletions(-) > > diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c > index bfae67c593c5..15ff196cbd6d 100644 > --- a/fs/btrfs/extent_io.c > +++ b/fs/btrfs/extent_io.c > @@ -6328,18 +6328,16 @@ static int release_extent_buffer(struct extent_buffer *eb) > void free_extent_buffer(struct extent_buffer *eb) > { > int refs; > - int old; > if (!eb) > return; > > + refs = atomic_read(&eb->refs); > while (1) { > - refs = atomic_read(&eb->refs); > if ((!test_bit(EXTENT_BUFFER_UNMAPPED, &eb->bflags) && refs <= 3) > || (test_bit(EXTENT_BUFFER_UNMAPPED, &eb->bflags) && > refs == 1)) > break; > - old = atomic_cmpxchg(&eb->refs, refs, refs - 1); > - if (old == refs) > + if (atomic_try_cmpxchg(&eb->refs, &refs, refs - 1)) > return; > } > > -- > 2.37.1 >
On Tue, Aug 09, 2022 at 06:36:33PM +0200, Uros Bizjak wrote: > Use `atomic_try_cmpxchg(ptr, &old, new)` instead of > `atomic_cmpxchg(ptr, old, new) == old` in free_extent_buffer. This > has two benefits: > > - The x86 cmpxchg instruction returns success in the ZF flag, so this > change saves a compare after cmpxchg, as well as a related move > instruction in the front of cmpxchg. > > - atomic_try_cmpxchg implicitly assigns the *ptr value to &old when > cmpxchg fails, enabling further code simplifications. > > This patch has no functional change. > > Cc: Chris Mason <clm@fb.com> > Cc: Josef Bacik <josef@toxicpanda.com> > Cc: David Sterba <dsterba@suse.com> > Signed-off-by: Uros Bizjak <ubizjak@gmail.com> It's not in a performance critical code but resulting code is slightly more efficient. Added to misc-next, thanks.
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c index bfae67c593c5..15ff196cbd6d 100644 --- a/fs/btrfs/extent_io.c +++ b/fs/btrfs/extent_io.c @@ -6328,18 +6328,16 @@ static int release_extent_buffer(struct extent_buffer *eb) void free_extent_buffer(struct extent_buffer *eb) { int refs; - int old; if (!eb) return; + refs = atomic_read(&eb->refs); while (1) { - refs = atomic_read(&eb->refs); if ((!test_bit(EXTENT_BUFFER_UNMAPPED, &eb->bflags) && refs <= 3) || (test_bit(EXTENT_BUFFER_UNMAPPED, &eb->bflags) && refs == 1)) break; - old = atomic_cmpxchg(&eb->refs, refs, refs - 1); - if (old == refs) + if (atomic_try_cmpxchg(&eb->refs, &refs, refs - 1)) return; }
Use `atomic_try_cmpxchg(ptr, &old, new)` instead of `atomic_cmpxchg(ptr, old, new) == old` in free_extent_buffer. This has two benefits: - The x86 cmpxchg instruction returns success in the ZF flag, so this change saves a compare after cmpxchg, as well as a related move instruction in the front of cmpxchg. - atomic_try_cmpxchg implicitly assigns the *ptr value to &old when cmpxchg fails, enabling further code simplifications. This patch has no functional change. Cc: Chris Mason <clm@fb.com> Cc: Josef Bacik <josef@toxicpanda.com> Cc: David Sterba <dsterba@suse.com> Signed-off-by: Uros Bizjak <ubizjak@gmail.com> --- fs/btrfs/extent_io.c | 6 ++---- 1 file changed, 2 insertions(+), 4 deletions(-)