mpam/resctrl: Fix a deadlock due to inaccurate reference

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I9PVOZ



--------------------------------

Found a deadlock issue in mpam which is already fixed in x86 resctrl by
commit 334b0f4e ("x86/resctrl: Fix a deadlock due to inaccurate
reference").

There is a race condition which results in a deadlock when rmdir and
mkdir concurrently:

Thread 1: rmdir /sys/fs/resctrl/p1
Thread 2: mkdir /sys/fs/resctrl/p1/mon_groups/m1

Thread 1 is deleting control group "p1". Holding rdtgroup_mutex,
kernfs_remove() removes all kernfs nodes under directory "p1"
recursively, then waits for sub kernfs node "mon_groups" to drop active
reference.

Thread 2 is trying to create a subdirectory "m1" in the "mon_groups"
directory. The wrapper kernfs_iop_mkdir() takes an active reference to
the "mon_groups" directory but the code drops the active reference to
the parent directory "p1" instead.

As a result, Thread 1 is blocked on waiting for active reference to drop
and never release rdtgroup_mutex, while Thread 2 is also blocked on
trying to get rdtgroup_mutex.

Thread 1 (rdtgroup_rmdir)   Thread 2 (rdtgroup_mkdir)
(rmdir /sys/fs/resctrl/p1)  (mkdir /sys/fs/resctrl/p1/mon_groups/m1)
-------------------------   -------------------------
                            kernfs_iop_mkdir
                              /*
                               * kn: "m1", parent_kn: "mon_groups",
                               * prgrp_kn: parent_kn->parent: "p1",
                               *
                               * "mon_groups", parent_kn->active++: 1
                               */
                              kernfs_get_active(parent_kn)
kernfs_iop_rmdir
  /* "p1", kn->active++ */
  kernfs_get_active(kn)

  rdtgroup_kn_lock_live
    atomic_inc(&rdtgrp->waitcount)
    /* "p1", kn->active-- */
    kernfs_break_active_protection(kn)
    mutex_lock

  rdtgroup_rmdir_ctrl
    free_all_child_rdtgrp
      sentry->flags = RDT_DELETED

    rdtgroup_ctrl_remove
      rdtgrp->flags = RDT_DELETED
      kernfs_get(kn)
      kernfs_remove(rdtgrp->kn)
        __kernfs_remove
          /* "mon_groups", sub_kn */
          atomic_add(KN_DEACTIVATED_BIAS, &sub_kn->active)
          kernfs_drain(sub_kn)
            /*
             * sub_kn->active == KN_DEACTIVATED_BIAS + 1,
             * waiting on sub_kn->active to drop, but it
             * never drops in Thread 2 which is blocked
             * on getting rdtgroup_mutex.
             */
Thread 1 hangs here ---->
            wait_event(sub_kn->active == KN_DEACTIVATED_BIAS)
            ...
                              rdtgroup_mkdir
                                rdtgroup_mkdir_mon(parent_kn, prgrp_kn)
                                  mkdir_rdt_prepare(parent_kn, prgrp_kn)
                                    rdtgroup_kn_lock_live(prgrp_kn)
                                      atomic_inc(&rdtgrp->waitcount)
                                      /*
                                       * "p1", prgrp_kn->active--
                                       *
                                       * The active reference on "p1" is
                                       * dropped, but not matching the
                                       * actual active reference taken
                                       * on "mon_groups", thus causing
                                       * Thread 1 to wait forever while
                                       * holding rdtgroup_mutex.
                                       */
                                      kernfs_break_active_protection(
                                                               prgrp_kn)
                                      /*
                                       * Trying to get rdtgroup_mutex
                                       * which is held by Thread 1.
                                       */
Thread 2 hangs here ---->             mutex_lock
                                      ...

The problem is that the creation of a subdirectory in the "mon_groups"
directory incorrectly releases the active protection of its parent
directory instead of itself before it starts waiting for rdtgroup_mutex.
This is triggered by the rdtgroup_mkdir() flow calling
rdtgroup_kn_lock_live()/rdtgroup_kn_unlock() with kernfs node of the
parent control group ("p1") as argument. It should be called with kernfs
node "mon_groups" instead. What is currently missing is that the
kn->priv of "mon_groups" is NULL instead of pointing to the rdtgrp.

Fix it by pointing kn->priv to rdtgrp when "mon_groups" is created. Then
it could be passed to rdtgroup_kn_lock_live()/rdtgroup_kn_unlock()
instead. And then it operates on the same rdtgroup structure but handles
the active reference of kernfs node "mon_groups" to prevent deadlock.
The same changes are also made to the "mon_data" directories.

This patch involved associating the private data of the "mon_groups"
and "mon_data" directories to the resource group to which they belong
instead of NULL as before.

Improving the directory removal safeguard to ensure that
subdirectories of the resctrl root directory can only be removed if they
are a child of the resctrl filesystem's root _and_ not associated with
the default resource group.

This patch results in some unused function parameters that will be
cleaned up in follow-up patch, this patch focuses on the fix only in
support of backport efforts.

Fixes: 6065f551 ("resctrlfs: init support resctrlfs")
Signed-off-by: Yu Liao <liaoyu15@huawei.com>