From: Marco Elver <elver@xxxxxxxxxx> [ Upstream commit 35fd7a637c42bb54ba4608f4d40ae6e55fc88781 ] The counters obj_pool_free, and obj_nr_tofree, and the flag obj_freeing are read locklessly outside the pool_lock critical sections. If read with plain accesses, this would result in data races. This is addressed as follows: * reads outside critical sections become READ_ONCE()s (pairing with WRITE_ONCE()s added); * writes become WRITE_ONCE()s (pairing with READ_ONCE()s added); since writes happen inside critical sections, only the write and not the read of RMWs needs to be atomic, thus WRITE_ONCE(var, var +/- X) is sufficient. The data races were reported by KCSAN: BUG: KCSAN: data-race in __free_object / fill_pool write to 0xffffffff8beb04f8 of 4 bytes by interrupt on cpu 1: __free_object+0x1ee/0x8e0 lib/debugobjects.c:404 __debug_check_no_obj_freed+0x199/0x330 lib/debugobjects.c:969 debug_check_no_obj_freed+0x3c/0x44 lib/debugobjects.c:994 slab_free_hook mm/slub.c:1422 [inline] read to 0xffffffff8beb04f8 of 4 bytes by task 1 on cpu 2: fill_pool+0x3d/0x520 lib/debugobjects.c:135 __debug_object_init+0x3c/0x810 lib/debugobjects.c:536 debug_object_init lib/debugobjects.c:591 [inline] debug_object_activate+0x228/0x320 lib/debugobjects.c:677 debug_rcu_head_queue kernel/rcu/rcu.h:176 [inline] BUG: KCSAN: data-race in __debug_object_init / fill_pool read to 0xffffffff8beb04f8 of 4 bytes by task 10 on cpu 6: fill_pool+0x3d/0x520 lib/debugobjects.c:135 __debug_object_init+0x3c/0x810 lib/debugobjects.c:536 debug_object_init_on_stack+0x39/0x50 lib/debugobjects.c:606 init_timer_on_stack_key kernel/time/timer.c:742 [inline] write to 0xffffffff8beb04f8 of 4 bytes by task 1 on cpu 3: alloc_object lib/debugobjects.c:258 [inline] __debug_object_init+0x717/0x810 lib/debugobjects.c:544 debug_object_init lib/debugobjects.c:591 [inline] debug_object_activate+0x228/0x320 lib/debugobjects.c:677 debug_rcu_head_queue kernel/rcu/rcu.h:176 [inline] BUG: KCSAN: data-race in free_obj_work / free_object read to 0xffffffff9140c190 of 4 bytes by task 10 on cpu 6: free_object+0x4b/0xd0 lib/debugobjects.c:426 debug_object_free+0x190/0x210 lib/debugobjects.c:824 destroy_timer_on_stack kernel/time/timer.c:749 [inline] write to 0xffffffff9140c190 of 4 bytes by task 93 on cpu 1: free_obj_work+0x24f/0x480 lib/debugobjects.c:313 process_one_work+0x454/0x8d0 kernel/workqueue.c:2264 worker_thread+0x9a/0x780 kernel/workqueue.c:2410 Reported-by: Qian Cai <cai@xxxxxx> Signed-off-by: Marco Elver <elver@xxxxxxxxxx> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Link: https://lore.kernel.org/r/20200116185529.11026-1-elver@xxxxxxxxxx Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> --- lib/debugobjects.c | 46 +++++++++++++++++++++++++--------------------- 1 file changed, 25 insertions(+), 21 deletions(-) diff --git a/lib/debugobjects.c b/lib/debugobjects.c index 61261195f5b60..48054dbf1b51f 100644 --- a/lib/debugobjects.c +++ b/lib/debugobjects.c @@ -132,14 +132,18 @@ static void fill_pool(void) struct debug_obj *obj; unsigned long flags; - if (likely(obj_pool_free >= debug_objects_pool_min_level)) + if (likely(READ_ONCE(obj_pool_free) >= debug_objects_pool_min_level)) return; /* * Reuse objs from the global free list; they will be reinitialized * when allocating. + * + * Both obj_nr_tofree and obj_pool_free are checked locklessly; the + * READ_ONCE()s pair with the WRITE_ONCE()s in pool_lock critical + * sections. */ - while (obj_nr_tofree && (obj_pool_free < obj_pool_min_free)) { + while (READ_ONCE(obj_nr_tofree) && (READ_ONCE(obj_pool_free) < obj_pool_min_free)) { raw_spin_lock_irqsave(&pool_lock, flags); /* * Recheck with the lock held as the worker thread might have @@ -148,9 +152,9 @@ static void fill_pool(void) while (obj_nr_tofree && (obj_pool_free < obj_pool_min_free)) { obj = hlist_entry(obj_to_free.first, typeof(*obj), node); hlist_del(&obj->node); - obj_nr_tofree--; + WRITE_ONCE(obj_nr_tofree, obj_nr_tofree - 1); hlist_add_head(&obj->node, &obj_pool); - obj_pool_free++; + WRITE_ONCE(obj_pool_free, obj_pool_free + 1); } raw_spin_unlock_irqrestore(&pool_lock, flags); } @@ -158,7 +162,7 @@ static void fill_pool(void) if (unlikely(!obj_cache)) return; - while (obj_pool_free < debug_objects_pool_min_level) { + while (READ_ONCE(obj_pool_free) < debug_objects_pool_min_level) { struct debug_obj *new[ODEBUG_BATCH_SIZE]; int cnt; @@ -174,7 +178,7 @@ static void fill_pool(void) while (cnt) { hlist_add_head(&new[--cnt]->node, &obj_pool); debug_objects_allocated++; - obj_pool_free++; + WRITE_ONCE(obj_pool_free, obj_pool_free + 1); } raw_spin_unlock_irqrestore(&pool_lock, flags); } @@ -236,7 +240,7 @@ alloc_object(void *addr, struct debug_bucket *b, struct debug_obj_descr *descr) obj = __alloc_object(&obj_pool); if (obj) { obj_pool_used++; - obj_pool_free--; + WRITE_ONCE(obj_pool_free, obj_pool_free - 1); /* * Looking ahead, allocate one batch of debug objects and @@ -255,7 +259,7 @@ alloc_object(void *addr, struct debug_bucket *b, struct debug_obj_descr *descr) &percpu_pool->free_objs); percpu_pool->obj_free++; obj_pool_used++; - obj_pool_free--; + WRITE_ONCE(obj_pool_free, obj_pool_free - 1); } } @@ -309,8 +313,8 @@ static void free_obj_work(struct work_struct *work) obj = hlist_entry(obj_to_free.first, typeof(*obj), node); hlist_del(&obj->node); hlist_add_head(&obj->node, &obj_pool); - obj_pool_free++; - obj_nr_tofree--; + WRITE_ONCE(obj_pool_free, obj_pool_free + 1); + WRITE_ONCE(obj_nr_tofree, obj_nr_tofree - 1); } raw_spin_unlock_irqrestore(&pool_lock, flags); return; @@ -324,7 +328,7 @@ static void free_obj_work(struct work_struct *work) if (obj_nr_tofree) { hlist_move_list(&obj_to_free, &tofree); debug_objects_freed += obj_nr_tofree; - obj_nr_tofree = 0; + WRITE_ONCE(obj_nr_tofree, 0); } raw_spin_unlock_irqrestore(&pool_lock, flags); @@ -375,10 +379,10 @@ static void __free_object(struct debug_obj *obj) obj_pool_used--; if (work) { - obj_nr_tofree++; + WRITE_ONCE(obj_nr_tofree, obj_nr_tofree + 1); hlist_add_head(&obj->node, &obj_to_free); if (lookahead_count) { - obj_nr_tofree += lookahead_count; + WRITE_ONCE(obj_nr_tofree, obj_nr_tofree + lookahead_count); obj_pool_used -= lookahead_count; while (lookahead_count) { hlist_add_head(&objs[--lookahead_count]->node, @@ -396,15 +400,15 @@ static void __free_object(struct debug_obj *obj) for (i = 0; i < ODEBUG_BATCH_SIZE; i++) { obj = __alloc_object(&obj_pool); hlist_add_head(&obj->node, &obj_to_free); - obj_pool_free--; - obj_nr_tofree++; + WRITE_ONCE(obj_pool_free, obj_pool_free - 1); + WRITE_ONCE(obj_nr_tofree, obj_nr_tofree + 1); } } } else { - obj_pool_free++; + WRITE_ONCE(obj_pool_free, obj_pool_free + 1); hlist_add_head(&obj->node, &obj_pool); if (lookahead_count) { - obj_pool_free += lookahead_count; + WRITE_ONCE(obj_pool_free, obj_pool_free + lookahead_count); obj_pool_used -= lookahead_count; while (lookahead_count) { hlist_add_head(&objs[--lookahead_count]->node, @@ -423,7 +427,7 @@ static void __free_object(struct debug_obj *obj) static void free_object(struct debug_obj *obj) { __free_object(obj); - if (!obj_freeing && obj_nr_tofree) { + if (!READ_ONCE(obj_freeing) && READ_ONCE(obj_nr_tofree)) { WRITE_ONCE(obj_freeing, true); schedule_delayed_work(&debug_obj_work, ODEBUG_FREE_WORK_DELAY); } @@ -982,7 +986,7 @@ static void __debug_check_no_obj_freed(const void *address, unsigned long size) debug_objects_maxchecked = objs_checked; /* Schedule work to actually kmem_cache_free() objects */ - if (!obj_freeing && obj_nr_tofree) { + if (!READ_ONCE(obj_freeing) && READ_ONCE(obj_nr_tofree)) { WRITE_ONCE(obj_freeing, true); schedule_delayed_work(&debug_obj_work, ODEBUG_FREE_WORK_DELAY); } @@ -1008,12 +1012,12 @@ static int debug_stats_show(struct seq_file *m, void *v) seq_printf(m, "max_checked :%d\n", debug_objects_maxchecked); seq_printf(m, "warnings :%d\n", debug_objects_warnings); seq_printf(m, "fixups :%d\n", debug_objects_fixups); - seq_printf(m, "pool_free :%d\n", obj_pool_free + obj_percpu_free); + seq_printf(m, "pool_free :%d\n", READ_ONCE(obj_pool_free) + obj_percpu_free); seq_printf(m, "pool_pcp_free :%d\n", obj_percpu_free); seq_printf(m, "pool_min_free :%d\n", obj_pool_min_free); seq_printf(m, "pool_used :%d\n", obj_pool_used - obj_percpu_free); seq_printf(m, "pool_max_used :%d\n", obj_pool_max_used); - seq_printf(m, "on_free_list :%d\n", obj_nr_tofree); + seq_printf(m, "on_free_list :%d\n", READ_ONCE(obj_nr_tofree)); seq_printf(m, "objs_allocated:%d\n", debug_objects_allocated); seq_printf(m, "objs_freed :%d\n", debug_objects_freed); return 0; -- 2.20.1
