cgroup: llist: avoid memory tears for llist_node

cgroup: llist: avoid memory tears for llist_node

Before the commit 36df6e3dbd7e ("cgroup: make css_rstat_updated nmi
safe"), the struct llist_node is expected to be private to the one
inserting the node to the lockless list or the one removing the node
from the lockless list. After the mentioned commit, the llist_node in
the rstat code is per-cpu shared between the stacked contexts i.e.
process, softirq, hardirq & nmi. It is possible the compiler may tear
the loads or stores of llist_node. Let's avoid that.

KCSAN reported the following race:

 Reported by Kernel Concurrency Sanitizer on:
 CPU: 60 UID: 0 PID: 5425 ... 6.16.0-rc3-next-20250626 #1 NONE
 Tainted: [E]=UNSIGNED_MODULE
 Hardware name: ...
 ==================================================================
 ==================================================================
 BUG: KCSAN: data-race in css_rstat_flush / css_rstat_updated
 write to 0xffffe8fffe1c85f0 of 8 bytes by task 1061 on cpu 1:
  css_rstat_flush+0x1b8/0xeb0
  __mem_cgroup_flush_stats+0x184/0x190
  flush_memcg_stats_dwork+0x22/0x50
  process_one_work+0x335/0x630
  worker_thread+0x5f1/0x8a0
  kthread+0x197/0x340
  ret_from_fork+0xd3/0x110
  ret_from_fork_asm+0x11/0x20
 read to 0xffffe8fffe1c85f0 of 8 bytes by task 3551 on cpu 15:
  css_rstat_updated+0x81/0x180
  mod_memcg_lruvec_state+0x113/0x2d0
  __mod_lruvec_state+0x3d/0x50
  lru_add+0x21e/0x3f0
  folio_batch_move_lru+0x80/0x1b0
  __folio_batch_add_and_move+0xd7/0x160
  folio_add_lru_vma+0x42/0x50
  do_anonymous_page+0x892/0xe90
  __handle_mm_fault+0xfaa/0x1520
  handle_mm_fault+0xdc/0x350
  do_user_addr_fault+0x1dc/0x650
  exc_page_fault+0x5c/0x110
  asm_exc_page_fault+0x22/0x30
 value changed: 0xffffe8fffe18e0d0 -> 0xffffe8fffe1c85f0

$ ./scripts/faddr2line vmlinux css_rstat_flush+0x1b8/0xeb0
css_rstat_flush+0x1b8/0xeb0:
init_llist_node at include/linux/llist.h:86
(inlined by) llist_del_first_init at include/linux/llist.h:308
(inlined by) css_process_update_tree at kernel/cgroup/rstat.c:148
(inlined by) css_rstat_updated_list at kernel/cgroup/rstat.c:258
(inlined by) css_rstat_flush at kernel/cgroup/rstat.c:389

$ ./scripts/faddr2line vmlinux css_rstat_updated+0x81/0x180
css_rstat_updated+0x81/0x180:
css_rstat_updated at kernel/cgroup/rstat.c:90 (discriminator 1)

These are expected race and a simple READ_ONCE/WRITE_ONCE resolves these
reports. However let's add comments to explain the race and the need for
memory barriers if stronger guarantees are needed.

More specifically the rstat updater and the flusher can race and cause a
scenario where the stats updater skips adding the css to the lockless
list but the flusher might not see those updates done by the skipped
updater. This is benign race and the subsequent flusher will flush those
stats and at the moment there aren't any rstat users which are not fine
with this kind of race. However some future user might want more
stricter guarantee, so let's add appropriate comments to ease the job of
future users.

Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Fixes: 36df6e3dbd7e ("cgroup: make css_rstat_updated nmi safe")
Signed-off-by: Tejun Heo <tj@kernel.org>