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[SRU,K,4/6] sbitmap: Use atomic_long_try_cmpxchg in __sbitmap_queue_get_batch

Message ID 20230606102828.218014-5-gerald.yang@canonical.com
State New
Headers show
Series sbitmap: fix IO hung due to waiters not waken up | expand

Commit Message

Gerald Yang June 6, 2023, 10:28 a.m. UTC 
From: Uros Bizjak <ubizjak@gmail.com>

BugLink: https://bugs.launchpad.net/bugs/2022318

Use atomic_long_try_cmpxchg instead of
atomic_long_cmpxchg (*ptr, old, new) == old in __sbitmap_queue_get_batch.
x86 CMPXCHG instruction returns success in ZF flag, so this change
saves a compare after cmpxchg (and related move instruction in front
of cmpxchg).

Also, atomic_long_cmpxchg implicitly assigns old *ptr value to "old"
when cmpxchg fails, enabling further code simplifications, e.g.
an extra memory read can be avoided in the loop.

No functional change intended.

Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Link: https://lore.kernel.org/r/20220908151200.9993-1-ubizjak@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
(cherry picked from commit c35227d4e8cbc70a6622cc7cc5f8c3bff513f1fa)
Signed-off-by: Gerald Yang <gerald.yang@canonical.com>
---
 lib/sbitmap.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)
diff mbox series

Patch

diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 47cd8fb894ba..cbfd2e677d87 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -533,16 +533,16 @@  unsigned long __sbitmap_queue_get_batch(struct sbitmap_queue *sbq, int nr_tags,
 		nr = find_first_zero_bit(&map->word, map_depth);
 		if (nr + nr_tags <= map_depth) {
 			atomic_long_t *ptr = (atomic_long_t *) &map->word;
-			unsigned long val, ret;
+			unsigned long val;
 
 			get_mask = ((1UL << nr_tags) - 1) << nr;
+			val = READ_ONCE(map->word);
 			do {
-				val = READ_ONCE(map->word);
 				if ((val & ~get_mask) != val)
 					goto next;
-				ret = atomic_long_cmpxchg(ptr, val, get_mask | val);
-			} while (ret != val);
-			get_mask = (get_mask & ~ret) >> nr;
+			} while (!atomic_long_try_cmpxchg(ptr, &val,
+							  get_mask | val));
+			get_mask = (get_mask & ~val) >> nr;
 			if (get_mask) {
 				*offset = nr + (index << sb->shift);
 				update_alloc_hint_after_get(sb, depth, hint,