Re: [PATCH] mm: memcg: use READ_ONCE()/WRITE_ONCE() to access stock->nr_pages

[Roman Gushchin <roman.gushchin@linux.dev>]

On Wed, May 01, 2024 at 02:54:20AM -0700, Breno Leitao wrote:
> A memcg pointer in the per-cpu stock can be accessed by drain_all_stock()
> and consume_stock() in parallel, causing a potential race.
> 
> 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.

Btw there is an actual data race, which is periodically causing a needless
draining of a per-cpu stock.

> 
> 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().
> 
> Signed-off-by: Breno Leitao <leitao@debian.org>

Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>