cgroup: don't skip seq_open on write only opens on pidlist files

Currently, cgroup_pidlist_open() skips seq_open() and pidlist loading
if the file is opened write-only, which is a sensible optimization as
pidlist loading can be costly and there often are occasions where
tasks or cgroup.procs is opened write-only.  However, pidlist init and
release are planned to be moved to cgroup_pidlist_start/stop()
respectively which would make this optimization unnecessary.

This patch removes the optimization and always fully initializes
pidlist files regardless of open mode.  This will help moving pidlist
handling to start/stop by unifying rw paths and removes the need for
specifying cftype->release() in addition to .release in
cgroup_pidlist_operations as file->f_op is now always overridden.  As
pidlist files were the only user of cftype->release(), the next patch
will remove the method.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

kernel/cgroup.c

@@ -3781,12 +3781,7 @@ static void cgroup_release_pid_array(struct cgroup_pidlist *l)
 static int cgroup_pidlist_release(struct inode *inode, struct file *file)
 {
 	struct cgroup_pidlist *l;
-	if (!(file->f_mode & FMODE_READ))
-		return 0;
-	/*
-	 * the seq_file will only be initialized if the file was opened for
-	 * reading; hence we check if it's not null only in that case.
-	 */
+
 	l = ((struct seq_file *)file->private_data)->private;
 	cgroup_release_pid_array(l);
 	return seq_release(inode, file);
@@ -3811,10 +3806,6 @@ static int cgroup_pidlist_open(struct file *file, enum cgroup_filetype type)
 	struct cgroup_pidlist *l;
 	int retval;
 
-	/* Nothing to do for write-only files */
-	if (!(file->f_mode & FMODE_READ))
-		return 0;
-
 	/* have the array populated */
 	retval = pidlist_array_load(cgrp, type, &l);
 	if (retval)
@@ -3894,7 +3885,6 @@ static struct cftype cgroup_base_files[] = {
 		.name = "cgroup.procs",
 		.open = cgroup_procs_open,
 		.write_u64 = cgroup_procs_write,
-		.release = cgroup_pidlist_release,
 		.mode = S_IRUGO | S_IWUSR,
 	},
 	{
@@ -3919,7 +3909,6 @@ static struct cftype cgroup_base_files[] = {
 		.flags = CFTYPE_INSANE,		/* use "procs" instead */
 		.open = cgroup_tasks_open,
 		.write_u64 = cgroup_tasks_write,
-		.release = cgroup_pidlist_release,
 		.mode = S_IRUGO | S_IWUSR,
 	},
 	{