On Wed 01-05-24 02:54:20, 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. > > 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@xxxxxxxxxx> Acked-by: Michal Hocko <mhocko@xxxxxxxx> It is worth mentioning that the race is harmless. > --- > mm/memcontrol.c | 24 +++++++++++++++--------- > 1 file changed, 15 insertions(+), 9 deletions(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index fabce2b50c69..d3befe3b62fa 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -2310,6 +2310,7 @@ static void memcg_account_kmem(struct mem_cgroup *memcg, int nr_pages) > static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages) > { > struct memcg_stock_pcp *stock; > + unsigned int stock_pages; > unsigned long flags; > bool ret = false; > > @@ -2319,8 +2320,9 @@ static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages) > local_lock_irqsave(&memcg_stock.stock_lock, flags); > > stock = this_cpu_ptr(&memcg_stock); > - if (memcg == READ_ONCE(stock->cached) && stock->nr_pages >= nr_pages) { > - stock->nr_pages -= nr_pages; > + stock_pages = READ_ONCE(stock->nr_pages); > + if (memcg == READ_ONCE(stock->cached) && stock_pages >= nr_pages) { > + WRITE_ONCE(stock->nr_pages, stock_pages - nr_pages); > ret = true; > } > > @@ -2334,16 +2336,18 @@ static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages) > */ > static void drain_stock(struct memcg_stock_pcp *stock) > { > + unsigned int stock_pages = READ_ONCE(stock->nr_pages); > struct mem_cgroup *old = READ_ONCE(stock->cached); > > if (!old) > return; > > - if (stock->nr_pages) { > - page_counter_uncharge(&old->memory, stock->nr_pages); > + if (stock_pages) { > + page_counter_uncharge(&old->memory, stock_pages); > if (do_memsw_account()) > - page_counter_uncharge(&old->memsw, stock->nr_pages); > - stock->nr_pages = 0; > + page_counter_uncharge(&old->memsw, stock_pages); > + > + WRITE_ONCE(stock->nr_pages, 0); > } > > css_put(&old->css); > @@ -2380,6 +2384,7 @@ static void drain_local_stock(struct work_struct *dummy) > static void __refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages) > { > struct memcg_stock_pcp *stock; > + unsigned int stock_pages; > > stock = this_cpu_ptr(&memcg_stock); > if (READ_ONCE(stock->cached) != memcg) { /* reset if necessary */ > @@ -2387,9 +2392,10 @@ static void __refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages) > css_get(&memcg->css); > WRITE_ONCE(stock->cached, memcg); > } > - stock->nr_pages += nr_pages; > + stock_pages = READ_ONCE(stock->nr_pages) + nr_pages; > + WRITE_ONCE(stock->nr_pages, stock_pages); > > - if (stock->nr_pages > MEMCG_CHARGE_BATCH) > + if (stock_pages > MEMCG_CHARGE_BATCH) > drain_stock(stock); > } > > @@ -2428,7 +2434,7 @@ static void drain_all_stock(struct mem_cgroup *root_memcg) > > rcu_read_lock(); > memcg = READ_ONCE(stock->cached); > - if (memcg && stock->nr_pages && > + if (memcg && READ_ONCE(stock->nr_pages) && > mem_cgroup_is_descendant(memcg, root_memcg)) > flush = true; > else if (obj_stock_flush_required(stock, root_memcg)) > -- > 2.43.0 -- Michal Hocko SUSE Labs