[PATCH] sched: Do not bug in __sched_setscheduler() when pi is not used

From: Steven Rostedt
Date: Thu Mar 09 2017 - 10:18:56 EST

From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>

When priority inheritance was added back in 2.6.18 to sched_setscheduler, it
added a path to taking an rt-mutex wait_lock, which is not IRQ safe. As PI
is not a common occurrence, lockdep will likely never trigger if
sched_setscheduler was called from interrupt context. A BUG_ON() was added
to trigger if __sched_setscheduler() was ever called from interrupt context
because there was a possibility to take the wait_lock.

Today the wait_lock is irq safe, but the path to taking it in
sched_setscheduler() is the same as the path to taking it from normal
context. The wait_lock is taken with raw_spin_lock_irq() and released with
raw_spin_unlock_irq() which will indiscriminately enable interrupts,
which would be bad in interrupt context.

The problem is that normalize_rt_tasks, which is called by triggering the
sysrq nice-all-RT-tasks was changed to call __sched_setscheduler(), and this
is done from interrupt context!

Now __sched_setscheduler() takes a "pi" parameter that is used to know if
the priority inheritance should be called or not. As the BUG_ON() only cares
about calling the PI code, it should only bug if called from interrupt
context with the "pi" parameter set to true.

Link: http://lkml.kernel.org/r/20170308124654.10e598f2@xxxxxxxxxxxxxxxxxx

Reported-by: Laurent Dufour <ldufour@xxxxxxxxxxxxxxxxxx>
Tested-by: Laurent Dufour <ldufour@xxxxxxxxxxxxxxxxxx>
Fixes: dbc7f069b93a ("sched: Use replace normalize_task() with __sched_setscheduler()")
Cc: Andrew Morton <akpm@xxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
kernel/sched/core.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3b31fc0..7292fa9 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4129,8 +4129,8 @@ static int __sched_setscheduler(struct task_struct *p,
int queue_flags = DEQUEUE_SAVE | DEQUEUE_MOVE;
struct rq *rq;

- /* May grab non-irq protected spin_locks: */
- BUG_ON(in_interrupt());
+ /* The pi code expects interrupts enabled */
+ BUG_ON(pi && in_interrupt());
/* Double check policy once rq lock held: */
if (policy < 0) {