[PATCH 2/9] sched: Fix race in idle_balance()

Peter Zijlstra
2014-01-28T17:33:51Z

[-- Attachment #1: daniel_lezcano-2_sched-fix_race_in_idle_balance.patch --]
[-- Type: text/plain, Size: 1447 bytes --]

From: Daniel Lezcano <daniel.lezcano@linaro.org>

The scheduler main function 'schedule()' checks if there are no more tasks
on the runqueue. Then it checks if a task should be pulled in the current
runqueue in idle_balance() assuming it will go to idle otherwise.

But the idle_balance() releases the rq->lock in order to lookup in the sched
domains and takes the lock again right after. That opens a window where
another cpu may put a task in our runqueue, so we won't go to idle but
we have filled the idle_stamp, thinking we will.

This patch closes the window by checking if the runqueue has been modified
but without pulling a task after taking the lock again, so we won't go to idle
right after in the __schedule() function.

Cc: alex.shi@linaro.org
Cc: peterz@infradead.org
Cc: mingo@kernel.org
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
---
kernel/sched/fair.c | 7 +++++++
1 file changed, 7 insertions(+)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6417,6 +6417,13 @@ void idle_balance(struct rq *this_rq)

raw_spin_lock(&this_rq->lock);

+ /*
+ * While browsing the domains, we released the rq lock.
+ * A task could have be enqueued in the meantime
+ */
+ if (this_rq->nr_running && !pulled_task)
+ return;
+
if (pulled_task || time_after(jiffies, this_rq->next_balance)) {
/*
* We are going idle. next_balance may be set based on