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/20210830172953.207257-1-shakeelb@google.com
Signed-off-by: Shakeel Butt <shakeelb@google.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 81fa831..06ae007 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4839,9 +4839,9 @@ 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);
@@ -5268,12 +5268,12 @@ 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);