The knob for cgroup v1 memory controller: memory.oom_control
is not protected by any locking so it can be modified while it is used.
This is not an actual problem because races are unlikely.
But it is better to use READ_ONCE/WRITE_ONCE to prevent compiler from
doing anything funky.

The access of memcg->oom_kill_disable is lockless,
so it can be concurrently set at the same time as we are
trying to read it.

Signed-off-by: Yue Zhao <findns94@gmail.com>
---
 mm/memcontrol.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index dca895c66a9b..26605b2f51b1 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4515,7 +4515,7 @@ static int mem_cgroup_oom_control_read(struct seq_file *sf, void *v)
 {
 	struct mem_cgroup *memcg = mem_cgroup_from_seq(sf);
 
-	seq_printf(sf, "oom_kill_disable %d\n", memcg->oom_kill_disable);
+	seq_printf(sf, "oom_kill_disable %d\n", READ_ONCE(memcg->oom_kill_disable));
 	seq_printf(sf, "under_oom %d\n", (bool)memcg->under_oom);
 	seq_printf(sf, "oom_kill %lu\n",
 		   atomic_long_read(&memcg->memory_events[MEMCG_OOM_KILL]));
@@ -4531,7 +4531,7 @@ static int mem_cgroup_oom_control_write(struct cgroup_subsys_state *css,
 	if (mem_cgroup_is_root(memcg) || !((val == 0) || (val == 1)))
 		return -EINVAL;
 
-	memcg->oom_kill_disable = val;
+	WRITE_ONCE(memcg->oom_kill_disable, val);
 	if (!val)
 		memcg_oom_recover(memcg);
 
-- 
2.17.1

On Mon 06-03-23 23:41:37, Yue Zhao wrote:
> The knob for cgroup v1 memory controller: memory.oom_control
> is not protected by any locking so it can be modified while it is used.
> This is not an actual problem because races are unlikely.
> But it is better to use READ_ONCE/WRITE_ONCE to prevent compiler from
> doing anything funky.
> 
> The access of memcg->oom_kill_disable is lockless,
> so it can be concurrently set at the same time as we are
> trying to read it.
> 
> Signed-off-by: Yue Zhao <findns94@gmail.com>
> ---
>  mm/memcontrol.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index dca895c66a9b..26605b2f51b1 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4515,7 +4515,7 @@ static int mem_cgroup_oom_control_read(struct seq_file *sf, void *v)
>  {
>  	struct mem_cgroup *memcg = mem_cgroup_from_seq(sf);
>  
> -	seq_printf(sf, "oom_kill_disable %d\n", memcg->oom_kill_disable);
> +	seq_printf(sf, "oom_kill_disable %d\n", READ_ONCE(memcg->oom_kill_disable));
>  	seq_printf(sf, "under_oom %d\n", (bool)memcg->under_oom);
>  	seq_printf(sf, "oom_kill %lu\n",
>  		   atomic_long_read(&memcg->memory_events[MEMCG_OOM_KILL]));
> @@ -4531,7 +4531,7 @@ static int mem_cgroup_oom_control_write(struct cgroup_subsys_state *css,
>  	if (mem_cgroup_is_root(memcg) || !((val == 0) || (val == 1)))
>  		return -EINVAL;
>  
> -	memcg->oom_kill_disable = val;
> +	WRITE_ONCE(memcg->oom_kill_disable, val);
>  	if (!val)
>  		memcg_oom_recover(memcg);

Any specific reasons you haven't covered other accesses
(mem_cgroup_css_alloc, mem_cgroup_oom, mem_cgroup_oom_synchronize)?
>  
> -- 
> 2.17.1

-- 
Michal Hocko
SUSE Labs

On Tue, Mar 7, 2023 at 1:53 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Mon 06-03-23 23:41:37, Yue Zhao wrote:
> > The knob for cgroup v1 memory controller: memory.oom_control
> > is not protected by any locking so it can be modified while it is used.
> > This is not an actual problem because races are unlikely.
> > But it is better to use READ_ONCE/WRITE_ONCE to prevent compiler from
> > doing anything funky.
> >
> > The access of memcg->oom_kill_disable is lockless,
> > so it can be concurrently set at the same time as we are
> > trying to read it.
> >
> > Signed-off-by: Yue Zhao <findns94@gmail.com>
> > ---
> >  mm/memcontrol.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index dca895c66a9b..26605b2f51b1 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -4515,7 +4515,7 @@ static int mem_cgroup_oom_control_read(struct seq_file *sf, void *v)
> >  {
> >       struct mem_cgroup *memcg = mem_cgroup_from_seq(sf);
> >
> > -     seq_printf(sf, "oom_kill_disable %d\n", memcg->oom_kill_disable);
> > +     seq_printf(sf, "oom_kill_disable %d\n", READ_ONCE(memcg->oom_kill_disable));
> >       seq_printf(sf, "under_oom %d\n", (bool)memcg->under_oom);
> >       seq_printf(sf, "oom_kill %lu\n",
> >                  atomic_long_read(&memcg->memory_events[MEMCG_OOM_KILL]));
> > @@ -4531,7 +4531,7 @@ static int mem_cgroup_oom_control_write(struct cgroup_subsys_state *css,
> >       if (mem_cgroup_is_root(memcg) || !((val == 0) || (val == 1)))
> >               return -EINVAL;
> >
> > -     memcg->oom_kill_disable = val;
> > +     WRITE_ONCE(memcg->oom_kill_disable, val);
> >       if (!val)
> >               memcg_oom_recover(memcg);
>
> Any specific reasons you haven't covered other accesses
> (mem_cgroup_css_alloc, mem_cgroup_oom, mem_cgroup_oom_synchronize)?

Thanks for point this out, you are right, we should add
[READ|WRITE]_ONCE for all used places.
Let me create PATCH v3 later.
Also for the memcg->soft_limit, I will update as well.

> >
> > --
> > 2.17.1
>
> --
> Michal Hocko
> SUSE Labs