blob: faab0c87995670cff1254f63a5f3bc0304e6d713 [file] [log] [blame]
From d0d7cc7caa90de87ee98e2811027ef16be8a86c8 Mon Sep 17 00:00:00 2001
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Tue, 17 Mar 2020 11:04:09 -0700
Subject: [PATCH] mm: slub: be more careful about the double cmpxchg of
freelist
commit 5076190daded2197f62fe92cf69674488be44175 upstream.
This is just a cleanup addition to Jann's fix to properly update the
transaction ID for the slub slowpath in commit fd4d9c7d0c71 ("mm: slub:
add missing TID bump..").
The transaction ID is what protects us against any concurrent accesses,
but we should really also make sure to make the 'freelist' comparison
itself always use the same freelist value that we then used as the new
next free pointer.
Jann points out that if we do all of this carefully, we could skip the
transaction ID update for all the paths that only remove entries from
the lists, and only update the TID when adding entries (to avoid the ABA
issue with cmpxchg and list handling re-adding a previously seen value).
But this patch just does the "make sure to cmpxchg the same value we
used" rather than then try to be clever.
Acked-by: Jann Horn <jannh@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
diff --git a/mm/slub.c b/mm/slub.c
index 7a89fbb61e59..1d9edbb02ad1 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2967,11 +2967,13 @@ static __always_inline void do_slab_free(struct kmem_cache *s,
barrier();
if (likely(page == c->page)) {
- set_freepointer(s, tail_obj, c->freelist);
+ void **freelist = READ_ONCE(c->freelist);
+
+ set_freepointer(s, tail_obj, freelist);
if (unlikely(!this_cpu_cmpxchg_double(
s->cpu_slab->freelist, s->cpu_slab->tid,
- c->freelist, tid,
+ freelist, tid,
head, next_tid(tid)))) {
note_cmpxchg_failure("slab_free", s, tid);
--
2.7.4