In slab_alloc_node() and do_slab_free() fastpaths we need to guarantee that
our kmem_cache_cpu pointer is from the same cpu as the tid value. Currently
that's done by reading the tid first using this_cpu_read(), then the
kmem_cache_cpu pointer and verifying we read the same tid using the pointer and
plain READ_ONCE().
This can be simplified to just fetching kmem_cache_cpu pointer and then reading
tid using the pointer. That guarantees they are from the same cpu. We don't
need to read the tid using this_cpu_read() because the value will be validated
by this_cpu_cmpxchg_double(), making sure we are on the correct cpu and the
freelist didn't change by anyone preempting us since reading the tid.
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
mm/slub.c | 22 +++++++++-------------
1 file changed, 9 insertions(+), 13 deletions(-)
diff --git a/mm/slub.c b/mm/slub.c
index 83ad64c1d9da..7b4cdc59b9ff 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2840,15 +2840,14 @@ static __always_inline void *slab_alloc_node(struct kmem_cache *s,
* reading from one cpu area. That does not matter as long
* as we end up on the original cpu again when doing the cmpxchg.
*
- * We should guarantee that tid and kmem_cache are retrieved on
- * the same cpu. It could be different if CONFIG_PREEMPTION so we need
- * to check if it is matched or not.
+ * We must guarantee that tid and kmem_cache_cpu are retrieved on the
+ * same cpu. We read first the kmem_cache_cpu pointer and use it to read
+ * the tid. If we are preempted and switched to another cpu between the
+ * two reads, it's OK as the two are still associated with the same cpu
+ * and cmpxchg later will validate the cpu.
*/
- do {
- tid = this_cpu_read(s->cpu_slab->tid);
- c = raw_cpu_ptr(s->cpu_slab);
- } while (IS_ENABLED(CONFIG_PREEMPTION) &&
- unlikely(tid != READ_ONCE(c->tid)));
+ c = raw_cpu_ptr(s->cpu_slab);
+ tid = READ_ONCE(c->tid);
/*
* Irqless object alloc/free algorithm used here depends on sequence
@@ -3122,11 +3121,8 @@ static __always_inline void do_slab_free(struct kmem_cache *s,
* data is retrieved via this pointer. If we are on the same cpu
* during the cmpxchg then the free will succeed.
*/
- do {
- tid = this_cpu_read(s->cpu_slab->tid);
- c = raw_cpu_ptr(s->cpu_slab);
- } while (IS_ENABLED(CONFIG_PREEMPTION) &&
- unlikely(tid != READ_ONCE(c->tid)));
+ c = raw_cpu_ptr(s->cpu_slab);
+ tid = READ_ONCE(c->tid);
/* Same with comment on barrier() in slab_alloc_node() */
barrier();
--
2.31.1
