[PATCH] cgroup: pids: use {READ,WRITE}_ONCE for pids->limit operations

[Aleksa Sarai <cyphar@cyphar.com>]

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

Fixes: commit 49b786ea146f ("cgroup: implement the PIDs subsystem")
Cc: stable@vger.kernel.org # v4.3+
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 kernel/cgroup/pids.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/cgroup/pids.c b/kernel/cgroup/pids.c
index 8e513a573fe9..a726e4a20177 100644
--- a/kernel/cgroup/pids.c
+++ b/kernel/cgroup/pids.c
@@ -152,7 +152,7 @@ static int pids_try_charge(struct pids_cgroup *pids, int num)
 		 * p->limit is %PIDS_MAX then we know that this test will never
 		 * fail.
 		 */
-		if (new > p->limit)
+		if (new > READ_ONCE(p->limit))
 			goto revert;
 	}
 
@@ -277,7 +277,7 @@ 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;
+	WRITE_ONCE(pids->limit, limit);
 	return nbytes;
 }
 
@@ -285,7 +285,7 @@ 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 = READ_ONCE(pids->limit);
 
 	if (limit >= PIDS_MAX)
 		seq_printf(sf, "%s\n", PIDS_MAX_STR);
-- 
2.23.0