mm: memcg: use READ_ONCE()/WRITE_ONCE() to access stock->nr_pages

mm: memcg: use READ_ONCE()/WRITE_ONCE() to access stock->nr_pages

A memcg pointer in the per-cpu stock can be accessed by
drain_all_stock() and consume_stock() in parallel, causing a potential
race, which is believed to e harmless.

KCSAN shows this data-race clearly in the splat below:

	BUG: KCSAN: data-race in drain_all_stock.part.0 / try_charge_memcg

	write to 0xffff88903f8b0788 of 4 bytes by task 35901 on cpu 2:
	try_charge_memcg (mm/memcontrol.c:2323 mm/memcontrol.c:2746)
	__mem_cgroup_charge (mm/memcontrol.c:7287 mm/memcontrol.c:7301)
	do_anonymous_page (mm/memory.c:1054 mm/memory.c:4375 mm/memory.c:4433)
	__handle_mm_fault (mm/memory.c:3878 mm/memory.c:5300 mm/memory.c:5441)
	handle_mm_fault (mm/memory.c:5606)
	do_user_addr_fault (arch/x86/mm/fault.c:1363)
	exc_page_fault (./arch/x86/include/asm/irqflags.h:37
		        ./arch/x86/include/asm/irqflags.h:72
			arch/x86/mm/fault.c:1513
			arch/x86/mm/fault.c:1563)
	asm_exc_page_fault (./arch/x86/include/asm/idtentry.h:623)

	read to 0xffff88903f8b0788 of 4 bytes by task 287 on cpu 27:
	drain_all_stock.part.0 (mm/memcontrol.c:2433)
	mem_cgroup_css_offline (mm/memcontrol.c:5398 mm/memcontrol.c:5687)
	css_killed_work_fn (kernel/cgroup/cgroup.c:5521 kernel/cgroup/cgroup.c:5794)
	process_one_work (kernel/workqueue.c:3254)
	worker_thread (kernel/workqueue.c:3329 kernel/workqueue.c:3416)
	kthread (kernel/kthread.c:388)
	ret_from_fork (arch/x86/kernel/process.c:147)
	ret_from_fork_asm (arch/x86/entry/entry_64.S:257)

	value changed: 0x00000014 -> 0x00000013

This happens because drain_all_stock() is reading stock->nr_pages, while
consume_stock() might be updating the same address, causing a potential
data-race.

Make the shared addresses bulletproof regarding to reads and writes,
similarly to what stock->cached_objcg and stock->cached.
Annotate all accesses to stock->nr_pages with READ_ONCE()/WRITE_ONCE().

Link: https://lkml.kernel.org/r/20240501095420.679208-1-leitao@debian.org
Signed-off-by: Breno Leitao <leitao@debian.org>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>