mm, slub: simplify kmem_cache_cpu and tid setup

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@suse.cz>
Acked-by: Mel Gorman <mgorman@techsingularity.net>

diff --git a/mm/slub.c b/mm/slub.c
index 9607ce3..c0dc596 100644 (file)
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2882,15 +2882,14 @@ redo:
         * 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
@@ -3164,11 +3163,8 @@ redo:
         * 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();