[114/212] memcg: make memcg->event_list_lock irqsafe

Commit Message

Andrew Morton Sept. 2, 2021, 9:56 p.m. UTC

From: Shakeel Butt <[e-mail]>
Subject: 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

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Shakeel Butt <[e-mail]>
Cc: Tejun Heo <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 mm/memcontrol.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

Patch

--- a/mm/memcontrol.c~memcg-make-memcg-event_list_lock-irqsafe
+++ a/mm/memcontrol.c
@@ -4839,9 +4839,9 @@  static ssize_t memcg_write_event_control
 
 	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);
@@ -5268,12 +5268,12 @@  static void mem_cgroup_css_offline(struc
 	 * 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);