cgroup: release cgroup_mutex over file removals

Now that cftypes and all tree modification operations are protected by
cgroup_tree_mutex, we can drop cgroup_mutex while deleting files and
directories.  Drop cgroup_mutex over removals.

This doesn't make any noticeable difference now but is to help kernfs
conversion.  In kernfs, removals are sync points which drain in-flight
operations as those operations would grab cgroup_mutex, trying to
delete under cgroup_mutex would deadlock.  This can be resolved by
just holding the outer cgroup_tree_mutex which nests outside both
kernfs active reference and cgroup_mutex.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index cb20d12cb096a736a8c1aa12af729e7b37897ea3..d28cf75f33c1cb3ec39b701c79c050a005861e33 100644 (file)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -976,7 +976,9 @@ static int rebind_subsystems(struct cgroupfs_root *root,
         * Nothing can fail from this point on.  Remove files for the
         * removed subsystems and rebind each subsystem.
         */
+       mutex_unlock(&cgroup_mutex);
        cgroup_clear_dir(cgrp, removed_mask);
+       mutex_lock(&cgroup_mutex);
 
        for_each_subsys(ss, i) {
                unsigned long bit = 1UL << i;
@@ -2696,10 +2698,11 @@ static int cgroup_cfts_commit(struct cftype *cfts, bool is_add)
        u64 update_before;
        int ret = 0;
 
+       mutex_unlock(&cgroup_mutex);
+
        /* %NULL @cfts indicates abort and don't bother if @ss isn't attached */
        if (!cfts || ss->root == &cgroup_dummy_root ||
            !atomic_inc_not_zero(&sb->s_active)) {
-               mutex_unlock(&cgroup_mutex);
                mutex_unlock(&cgroup_tree_mutex);
                return 0;
        }
@@ -2723,18 +2726,15 @@ static int cgroup_cfts_commit(struct cftype *cfts, bool is_add)
                dput(prev);
                prev = cgrp->dentry;
 
-               mutex_unlock(&cgroup_mutex);
                mutex_unlock(&cgroup_tree_mutex);
                mutex_lock(&inode->i_mutex);
                mutex_lock(&cgroup_tree_mutex);
-               mutex_lock(&cgroup_mutex);
                if (cgrp->serial_nr < update_before && !cgroup_is_dead(cgrp))
                        ret = cgroup_addrm_files(cgrp, cfts, is_add);
                mutex_unlock(&inode->i_mutex);
                if (ret)
                        break;
        }
-       mutex_unlock(&cgroup_mutex);
        mutex_unlock(&cgroup_tree_mutex);
        dput(prev);
        deactivate_super(sb);
@@ -4387,10 +4387,13 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
        /*
         * Initiate massacre of all css's.  cgroup_destroy_css_killed()
         * will be invoked to perform the rest of destruction once the
-        * percpu refs of all css's are confirmed to be killed.
+        * percpu refs of all css's are confirmed to be killed.  This
+        * involves removing the subsystem's files, drop cgroup_mutex.
         */
+       mutex_unlock(&cgroup_mutex);
        for_each_css(css, ssid, cgrp)
                kill_css(css);
+       mutex_lock(&cgroup_mutex);
 
        /*
         * Mark @cgrp dead.  This prevents further task migration and child
@@ -4421,9 +4424,11 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
         * puts the base ref but we aren't quite done with @cgrp yet, so
         * hold onto it.
         */
+       mutex_unlock(&cgroup_mutex);
        cgroup_addrm_files(cgrp, cgroup_base_files, false);
        dget(d);
        cgroup_d_remove_dir(d);
+       mutex_lock(&cgroup_mutex);
 
        return 0;
 };