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