[Devel] [PATCH RHEL8 COMMIT] kernel/cgroup: Remove unnecessary cgroup_mutex lock. #PSBM-120670

[Konstantin Khorenko]

The commit is pushed to "branch-rh8-4.18.0-193.6.3.vz8.4.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh8-4.18.0-193.6.3.vz8.4.12
------>
commit 59490dde6e535e0447a8b27857b20bdd4b01e012
Author: Andrey Ryabinin <aryabinin at virtuozzo.com>
Date:   Wed Oct 14 15:41:00 2020 +0300

    kernel/cgroup: Remove unnecessary cgroup_mutex lock. #PSBM-120670
    
    Stopping container causes the lockdep to complain (see report bellow).
    We can avoid it simply by removing cgroup_mutex lock from
    cgroup_mark_ve_root(). I believe it's not needed there, it seems to be
    added just in case.
    
     WARNING: possible circular locking dependency detected
     4.18.0-193.6.3.vz8.4.6+debug #1 Not tainted
     ------------------------------------------------------
     vzctl/36606 is trying to acquire lock:
     ffff88814b195ca0 (kn->count#338){++++}, at: kernfs_remove_by_name_ns+0x40/0x80
    
     but task is already holding lock:
     ffffffff9cf75a90 (cgroup_mutex){+.+.}, at: cgroup_kn_lock_live+0x106/0x390
    
     which lock already depends on the new lock.
     the existing dependency chain (in reverse order) is:
    
     -> #2 (cgroup_mutex){+.+.}:
            __mutex_lock+0x163/0x13d0
            cgroup_mark_ve_root+0x1d/0x2e0
            ve_state_write+0xb81/0xdc0
            cgroup_file_write+0x2da/0x7a0
            kernfs_fop_write+0x255/0x410
            vfs_write+0x157/0x460
            ksys_write+0xb8/0x170
            do_syscall_64+0xa5/0x4d0
            entry_SYSCALL_64_after_hwframe+0x6a/0xdf
    
     -> #1 (&ve->op_sem){++++}:
            down_write+0xa0/0x3d0
            ve_state_write+0x6b/0xdc0
            cgroup_file_write+0x2da/0x7a0
            kernfs_fop_write+0x255/0x410
            vfs_write+0x157/0x460
            ksys_write+0xb8/0x170
            do_syscall_64+0xa5/0x4d0
            entry_SYSCALL_64_after_hwframe+0x6a/0xdf
    
     -> #0 (kn->count#338){++++}:
            __lock_acquire+0x22cb/0x48c0
            lock_acquire+0x14f/0x3b0
            __kernfs_remove+0x61e/0x810
            kernfs_remove_by_name_ns+0x40/0x80
            cgroup_addrm_files+0x531/0x940
            css_clear_dir+0xfb/0x200
            kill_css+0x8f/0x120
            cgroup_destroy_locked+0x246/0x5e0
            cgroup_rmdir+0x2f/0x2c0
            kernfs_iop_rmdir+0x131/0x1b0
            vfs_rmdir+0x142/0x3c0
            do_rmdir+0x2b2/0x340
            do_syscall_64+0xa5/0x4d0
            entry_SYSCALL_64_after_hwframe+0x6a/0xdf
    
     other info that might help us debug this:
    
     Chain exists of:
       kn->count#338 --> &ve->op_sem --> cgroup_mutex
    
      Possible unsafe locking scenario:
    
            CPU0                    CPU1
            ----                    ----
       lock(cgroup_mutex);
                                    lock(&ve->op_sem);
                                    lock(cgroup_mutex);
       lock(kn->count#338);
    
                    *** DEADLOCK ***
    
     4 locks held by vzctl/36606:
      #0: ffff88813c02c890 (sb_writers#7){.+.+}, at: mnt_want_write+0x3c/0xa0
      #1: ffff88814414ad48 (&type->i_mutex_dir_key#5/1){+.+.}, at: do_rmdir+0x23c/0x340
      #2: ffff88811d3054e8 (&type->i_mutex_dir_key#5){++++}, at: vfs_rmdir+0xb6/0x3c0
      #3: ffffffff9cf75a90 (cgroup_mutex){+.+.}, at: cgroup_kn_lock_live+0x106/0x390
    
     Call Trace:
      dump_stack+0x9a/0xf0
      check_noncircular+0x317/0x3c0
      __lock_acquire+0x22cb/0x48c0
      lock_acquire+0x14f/0x3b0
      __kernfs_remove+0x61e/0x810
      kernfs_remove_by_name_ns+0x40/0x80
      cgroup_addrm_files+0x531/0x940
      css_clear_dir+0xfb/0x200
      kill_css+0x8f/0x120
      cgroup_destroy_locked+0x246/0x5e0
      cgroup_rmdir+0x2f/0x2c0
      kernfs_iop_rmdir+0x131/0x1b0
      vfs_rmdir+0x142/0x3c0
      do_rmdir+0x2b2/0x340
      do_syscall_64+0xa5/0x4d0
      entry_SYSCALL_64_after_hwframe+0x6a/0xdf
    
    https://jira.sw.ru/browse/PSBM-120670
    Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
---
 kernel/cgroup/cgroup.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 8420f3547f1a..08137d43f3ab 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -1883,7 +1883,6 @@ void cgroup_mark_ve_root(struct ve_struct *ve)
 	struct css_set *cset;
 	struct cgroup *cgrp;
 
-	mutex_lock(&cgroup_mutex);
 	spin_lock_irq(&css_set_lock);
 
 	rcu_read_lock();
@@ -1899,7 +1898,6 @@ void cgroup_mark_ve_root(struct ve_struct *ve)
 	rcu_read_unlock();
 
 	spin_unlock_irq(&css_set_lock);
-	mutex_unlock(&cgroup_mutex);
 }
 
 static struct cgroup *cgroup_get_ve_root1(struct cgroup *cgrp)