path: root/mm
diff options
authorLinus Torvalds <torvalds@linux-foundation.org>2020-03-17 11:04:09 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2020-03-18 09:40:32 -0700
commit5076190daded2197f62fe92cf69674488be44175 (patch)
treee8d20e214d1c47b0ce3b8a45d525cee29675fd9d /mm
parentmm: slub: add missing TID bump in kmem_cache_alloc_bulk() (diff)
mm: slub: be more careful about the double cmpxchg of freelist
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>
Diffstat (limited to 'mm')
1 files changed, 4 insertions, 2 deletions
diff --git a/mm/slub.c b/mm/slub.c
index eae5bb47b22f..97580b41a24b 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2997,11 +2997,13 @@ redo:
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);