debugobjects: Fix various data races

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@lca.pw>
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20200116185529.11026-1-elver@google.com

diff --git a/lib/debugobjects.c b/lib/debugobjects.c
index 61261195f5b60b4c143adbca5967f979372798c5..48054dbf1b51fbd1bb6486bcb8b5de1786bc2304 100644 (file)
--- 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 @@ free_objs:
        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 @@ free_to_obj_pool:
        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 @@ free_to_obj_pool:
                        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 @@ free_to_obj_pool:
 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 @@ repeat:
                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;