cgroup: pids: use atomic64_t for pids->limit

BugLink: https://bugs.launchpad.net/bugs/1858489

commit a713af39 upstream.

Because pids->limit can be changed concurrently (but we don't want to
take a lock because it would be needlessly expensive), use atomic64_ts
instead.

Fixes: commit 49b786ea ("cgroup: implement the PIDs subsystem")
Cc: stable@vger.kernel.org # v4.3+
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

kernel/cgroup_pids.c

@@ -48,7 +48,7 @@ struct pids_cgroup {
 	 * %PIDS_MAX = (%PID_MAX_LIMIT + 1).
 	 */
 	atomic64_t			counter;
-	int64_t				limit;
+	atomic64_t			limit;
 };
 
 static struct pids_cgroup *css_pids(struct cgroup_subsys_state *css)
@@ -70,8 +70,8 @@ pids_css_alloc(struct cgroup_subsys_state *parent)
 	if (!pids)
 		return ERR_PTR(-ENOMEM);
 
-	pids->limit = PIDS_MAX;
 	atomic64_set(&pids->counter, 0);
+	atomic64_set(&pids->limit, PIDS_MAX);
 	return &pids->css;
 }
 
@@ -142,13 +142,14 @@ static int pids_try_charge(struct pids_cgroup *pids, int num)
 
 	for (p = pids; parent_pids(p); p = parent_pids(p)) {
 		int64_t new = atomic64_add_return(num, &p->counter);
+		int64_t limit = atomic64_read(&p->limit);
 
 		/*
 		 * Since new is capped to the maximum number of pid_t, if
 		 * p->limit is %PIDS_MAX then we know that this test will never
 		 * fail.
 		 */
-		if (new > p->limit)
+		if (new > limit)
 			goto revert;
 	}
 
@@ -262,7 +263,7 @@ static ssize_t pids_max_write(struct kernfs_open_file *of, char *buf,
 	 * Limit updates don't need to be mutex'd, since it isn't
 	 * critical that any racing fork()s follow the new limit.
 	 */
-	pids->limit = limit;
+	atomic64_set(&pids->limit, limit);
 	return nbytes;
 }
 
@@ -270,7 +271,7 @@ static int pids_max_show(struct seq_file *sf, void *v)
 {
 	struct cgroup_subsys_state *css = seq_css(sf);
 	struct pids_cgroup *pids = css_pids(css);
-	int64_t limit = pids->limit;
+	int64_t limit = atomic64_read(&pids->limit);
 
 	if (limit >= PIDS_MAX)
 		seq_printf(sf, "%s\n", PIDS_MAX_STR);