Commit f785a8f2 authored by Roman Gushchin's avatar Roman Gushchin Committed by Andrew Morton
Browse files

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

A memcg pointer in the percpu stock can be accessed by drain_all_stock()
from another cpu in a lockless way.  In theory it might lead to an issue,
similar to the one which has been discovered with stock->cached_objcg,
where the pointer was zeroed between the check for being NULL and
dereferencing.  In this case the issue is unlikely a real problem, but to
make it bulletproof and similar to stock->cached_objcg, let's annotate all
accesses to stock->cached with READ_ONCE()/WTRITE_ONCE().

Link: https://lkml.kernel.org/r/20230502160839.361544-2-roman.gushchin@linux.dev


Signed-off-by: default avatarRoman Gushchin <roman.gushchin@linux.dev>
Acked-by: default avatarShakeel Butt <shakeelb@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 3b8abb32

mm/memcontrol.c

+6 −6
Original line number Diff line number Diff line
@@ -2275,7 +2275,7 @@ 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 == stock->cached && stock->nr_pages >= nr_pages) {

	if (memcg == READ_ONCE(stock->cached) && stock->nr_pages >= nr_pages) {

		stock->nr_pages -= nr_pages;

		ret = true;

	}
@@ -2290,7 +2290,7 @@ static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages) 
 */

static void drain_stock(struct memcg_stock_pcp *stock)

{

	struct mem_cgroup *old = stock->cached;

	struct mem_cgroup *old = READ_ONCE(stock->cached);



	if (!old)

		return;
@@ -2303,7 +2303,7 @@ static void drain_stock(struct memcg_stock_pcp *stock) 
	}



	css_put(&old->css);

	stock->cached = NULL;

	WRITE_ONCE(stock->cached, NULL);

}



static void drain_local_stock(struct work_struct *dummy)
@@ -2338,10 +2338,10 @@ static void __refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages) 
	struct memcg_stock_pcp *stock;



	stock = this_cpu_ptr(&memcg_stock);

	if (stock->cached != memcg) { /* reset if necessary */

	if (READ_ONCE(stock->cached) != memcg) { /* reset if necessary */

		drain_stock(stock);

		css_get(&memcg->css);

		stock->cached = memcg;

		WRITE_ONCE(stock->cached, memcg);

	}

	stock->nr_pages += nr_pages;
@@ -2383,7 +2383,7 @@ static void drain_all_stock(struct mem_cgroup *root_memcg) 
		bool flush = false;



		rcu_read_lock();

		memcg = stock->cached;

		memcg = READ_ONCE(stock->cached);

		if (memcg && stock->nr_pages &&

		    mem_cgroup_is_descendant(memcg, root_memcg))

			flush = true;