linux-kernel - [PATCH] memcg: make memcg->event_list

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Date:   Mon, 30 Aug 2021 10:29:53 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, cgroups@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Shakeel Butt <shakeelb@...gle.com>
Subject: [PATCH] memcg: make memcg->event_list_lock irqsafe

The memcg->event_list_lock is usually taken in the normal context but
when the userspace closes the corresponding eventfd, eventfd_release
through memcg_event_wake takes memcg->event_list_lock with interrupts
disabled. This is not an issue on its own but it creates a nested
dependency from eventfd_ctx->wqh.lock to memcg->event_list_lock.

Independently, for unrelated eventfd, eventfd_signal() can be called in
the irq context, thus making eventfd_ctx->wqh.lock an irq lock. For
example, FPGA DFL driver, VHOST VPDA driver and couple of VFIO drivers.
This will force memcg->event_list_lock to be an irqsafe lock as well.

One way to break the nested dependency between eventfd_ctx->wqh.lock and
memcg->event_list_lock is to add an indirection. However the simplest
solution would be to make memcg->event_list_lock irqsafe. This is cgroup
v1 feature, is in maintenance and may get deprecated in near future. So,
no need to add more code.

BTW this has been discussed previously [1] but there weren't irq users
of eventfd_signal() at the time.

[1] https://www.spinics.net/lists/cgroups/msg06248.html

Signed-off-by: Shakeel Butt <shakeelb@...gle.com>
---
 mm/memcontrol.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 999e626f4111..a73689caee5d 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4851,9 +4851,9 @@ static ssize_t memcg_write_event_control(struct kernfs_open_file *of,

 	vfs_poll(efile.file, &event->pt);

-	spin_lock(&memcg->event_list_lock);
+	spin_lock_irq(&memcg->event_list_lock);
 	list_add(&event->list, &memcg->event_list);
-	spin_unlock(&memcg->event_list_lock);
+	spin_unlock_irq(&memcg->event_list_lock);

 	fdput(cfile);
 	fdput(efile);
@@ -5280,12 +5280,12 @@ static void mem_cgroup_css_offline(struct cgroup_subsys_state *css)
 	 * Notify userspace about cgroup removing only after rmdir of cgroup
 	 * directory to avoid race between userspace and kernelspace.
 	 */
-	spin_lock(&memcg->event_list_lock);
+	spin_lock_irq(&memcg->event_list_lock);
 	list_for_each_entry_safe(event, tmp, &memcg->event_list, list) {
 		list_del_init(&event->list);
 		schedule_work(&event->remove);
 	}
-	spin_unlock(&memcg->event_list_lock);
+	spin_unlock_irq(&memcg->event_list_lock);

 	page_counter_set_min(&memcg->memory, 0);
 	page_counter_set_low(&memcg->memory, 0);
-- 
2.33.0.259.gc128427fd7-goog