On Thu 03-10-19 15:44:29, Qian Cai wrote:
> Long time ago, there fixed a similar deadlock in show_slab_objects()
> [1]. However, it is apparently due to the commits like 01fb58bcba63
> ("slab: remove synchronous synchronize_sched() from memcg cache
> deactivation path") and 03afc0e25f7f ("slab: get_online_mems for
> kmem_cache_{create,destroy,shrink}"), this kind of deadlock is back by
> just reading files in /sys/kernel/slab will generate a lockdep splat
> below.
>
> Since the "mem_hotplug_lock" here is only to obtain a stable online node
> mask while racing with NUMA node hotplug, it is probably fine to do
> without it.
"It is probably fine" is not a proper justification. Please have a look
at my older email where I've exaplained why I believe it is safe.
> WARNING: possible circular locking dependency detected
> ------------------------------------------------------
I pressume the deadlock is real. If that is the case then Cc: stable and
Fixes tag would be really appreciated.
> Signed-off-by: Qian Cai <cai@xxxxxx>
Anyway, I do agree that this is the right thing to do. With the improved
changelog, fixed up the comment alignment feel free to add
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
> mm/slub.c | 11 +++++++++--
> 1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 42c1b3af3c98..922cdcf5758a 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -4838,7 +4838,15 @@ static ssize_t show_slab_objects(struct kmem_cache *s,
> }
> }
>
> - get_online_mems();
> +/*
> + * It is not possible to take "mem_hotplug_lock" here, as it has already held
> + * "kernfs_mutex" which could race with the lock order:
> + *
> + * mem_hotplug_lock->slab_mutex->kernfs_mutex
> + *
> + * In the worest case, it might be mis-calculated while doing NUMA node
> + * hotplug, but it shall be corrected by later reads of the same files.
> + */
> #ifdef CONFIG_SLUB_DEBUG
> if (flags & SO_ALL) {
> struct kmem_cache_node *n;
> @@ -4879,7 +4887,6 @@ static ssize_t show_slab_objects(struct kmem_cache *s,
> x += sprintf(buf + x, " N%d=%lu",
> node, nodes[node]);
> #endif
> - put_online_mems();
> kfree(nodes);
> return x + sprintf(buf + x, "\n");
> }
> --
> 1.8.3.1
>
--
Michal Hocko
SUSE Labs
