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Subject[PATCH 2/5] sched: Fix cfs_bandwidth misuse of hrtimer_expires_remaining
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hrtimer_expires_remaining does not take internal hrtimer locks and thus
must be guarded against concurrent __hrtimer_start_range_ns (but
returning HRTIMER_RESTART is safe). Use cfs_b->lock to make it safe.

Signed-off-by: Ben Segall <bsegall@google.com>
---
kernel/sched/fair.c | 14 +++++++++++---
1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 67c9ecf..b0eba47 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3294,7 +3294,13 @@ static const u64 min_bandwidth_expiration = 2 * NSEC_PER_MSEC;
/* how long we wait to gather additional slack before distributing */
static const u64 cfs_bandwidth_slack_period = 5 * NSEC_PER_MSEC;

-/* are we near the end of the current quota period? */
+/*
+ * Are we near the end of the current quota period?
+ *
+ * Requires cfs_b->lock for hrtimer_expires_remaining to be safe against the
+ * hrtimer base being cleared by __hrtimer_start_range_ns. This is possibly
+ * incorrect if we ever built with CPU_HOTPLUG.
+ */
static int runtime_refresh_within(struct cfs_bandwidth *cfs_b, u64 min_expire)
{
struct hrtimer *refresh_timer = &cfs_b->period_timer;
@@ -3370,10 +3376,12 @@ static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b)
u64 expires;

/* confirm we're still not at a refresh boundary */
- if (runtime_refresh_within(cfs_b, min_bandwidth_expiration))
+ raw_spin_lock(&cfs_b->lock);
+ if (runtime_refresh_within(cfs_b, min_bandwidth_expiration)) {
+ raw_spin_unlock(&cfs_b->lock);
return;
+ }

- raw_spin_lock(&cfs_b->lock);
if (cfs_b->quota != RUNTIME_INF && cfs_b->runtime > slice) {
runtime = cfs_b->runtime;
cfs_b->runtime = 0;


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