[PATCH v1] cgroup/cpuset: remove kernfs active break

[Chen Ridong]

From: Chen Ridong <chenridong@xxxxxxxxxx>

A warning was found:

WARNING: CPU: 10 PID: 3486953 at fs/kernfs/file.c:828
CPU: 10 PID: 3486953 Comm: rmdir Kdump: loaded Tainted: G
RIP: 0010:kernfs_should_drain_open_files+0x1a1/0x1b0
RSP: 0018:ffff8881107ef9e0 EFLAGS: 00010202
RAX: 0000000080000002 RBX: ffff888154738c00 RCX: dffffc0000000000
RDX: 0000000000000007 RSI: 0000000000000004 RDI: ffff888154738c04
RBP: ffff888154738c04 R08: ffffffffaf27fa15 R09: ffffed102a8e7180
R10: ffff888154738c07 R11: 0000000000000000 R12: ffff888154738c08
R13: ffff888750f8c000 R14: ffff888750f8c0e8 R15: ffff888154738ca0
FS:  00007f84cd0be740(0000) GS:ffff8887ddc00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000555f9fbe00c8 CR3: 0000000153eec001 CR4: 0000000000370ee0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 kernfs_drain+0x15e/0x2f0
 __kernfs_remove+0x165/0x300
 kernfs_remove_by_name_ns+0x7b/0xc0
 cgroup_rm_file+0x154/0x1c0
 cgroup_addrm_files+0x1c2/0x1f0
 css_clear_dir+0x77/0x110
 kill_css+0x4c/0x1b0
 cgroup_destroy_locked+0x194/0x380
 cgroup_rmdir+0x2a/0x140

It can be explained by:
rmdir 				echo 1 > cpuset.cpus
				kernfs_fop_write_iter // active=0
cgroup_rm_file
kernfs_remove_by_name_ns	kernfs_get_active // active=1
__kernfs_remove					  // active=0x80000002
kernfs_drain			cpuset_write_resmask
wait_event
//waiting (active == 0x80000001)
				kernfs_break_active_protection
				// active = 0x80000001
// continue
				kernfs_unbreak_active_protection
				// active = 0x80000002
...
kernfs_should_drain_open_files
// warning occurs
				kernfs_put_active

This warning is caused by 'kernfs_break_active_protection' when it is
writing to cpuset.cpus, and the cgroup is removed concurrently.

The commit 3a5a6d0c2b03 ("cpuset: don't nest cgroup_mutex inside
get_online_cpus()") made cpuset_hotplug_workfn asynchronous, which grabs
the cgroup_mutex. To avoid deadlock. the commit 76bb5ab8f6e3 ("cpuset:
break kernfs active protection in cpuset_write_resmask()") added
'kernfs_break_active_protection' in the cpuset_write_resmask. This could
lead to this warning.

After the commit 2125c0034c5d ("cgroup/cpuset: Make cpuset hotplug
processing synchronous"), the cpuset_write_resmask no longer needs to
wait the hotplug to finish, which means that cpuset_write_resmask won't
grab the cgroup_mutex. So the deadlock doesn't exist anymore. Therefore,
remove kernfs_break_active_protection operation in the
'cpuset_write_resmask'

Fixes: 76bb5ab8f6e3 ("cpuset: break kernfs active protection in cpuset_write_resmask()")
Reported-by: Ji Fa <jifa@xxxxxxxxxx>
Signed-off-by: Chen Ridong <chenridong@xxxxxxxxxx>
---
 kernel/cgroup/cpuset.c | 25 -------------------------
 1 file changed, 25 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 7ea559fb0cbf..0f910c828973 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -3124,29 +3124,6 @@ ssize_t cpuset_write_resmask(struct kernfs_open_file *of,
 	int retval = -ENODEV;
 
 	buf = strstrip(buf);
-
-	/*
-	 * CPU or memory hotunplug may leave @cs w/o any execution
-	 * resources, in which case the hotplug code asynchronously updates
-	 * configuration and transfers all tasks to the nearest ancestor
-	 * which can execute.
-	 *
-	 * As writes to "cpus" or "mems" may restore @cs's execution
-	 * resources, wait for the previously scheduled operations before
-	 * proceeding, so that we don't end up keep removing tasks added
-	 * after execution capability is restored.
-	 *
-	 * cpuset_handle_hotplug may call back into cgroup core asynchronously
-	 * via cgroup_transfer_tasks() and waiting for it from a cgroupfs
-	 * operation like this one can lead to a deadlock through kernfs
-	 * active_ref protection.  Let's break the protection.  Losing the
-	 * protection is okay as we check whether @cs is online after
-	 * grabbing cpuset_mutex anyway.  This only happens on the legacy
-	 * hierarchies.
-	 */
-	css_get(&cs->css);
-	kernfs_break_active_protection(of->kn);
-
 	cpus_read_lock();
 	mutex_lock(&cpuset_mutex);
 	if (!is_cpuset_online(cs))
@@ -3179,8 +3156,6 @@ ssize_t cpuset_write_resmask(struct kernfs_open_file *of,
 out_unlock:
 	mutex_unlock(&cpuset_mutex);
 	cpus_read_unlock();
-	kernfs_unbreak_active_protection(of->kn);
-	css_put(&cs->css);
 	flush_workqueue(cpuset_migrate_mm_wq);
 	return retval ?: nbytes;
 }
-- 
2.34.1