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Subject[PATCH 4.9 83/92] sched/core: Allow __sched_setscheduler() in interrupts when PI is not used
4.9-stable review patch.  If anyone has any objections, please let me know.


From: Steven Rostedt (VMware) <>

commit 896bbb2522587e3b8eb2a0d204d43ccc1042a00d upstream.

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.

Reported-by: Laurent Dufour <>
Tested-by: Laurent Dufour <>
Signed-off-by: Steven Rostedt (VMware) <>
Signed-off-by: Peter Zijlstra (Intel) <>
Cc: Andrew Morton <>
Cc: Linus Torvalds <>
Cc: Peter Zijlstra <>
Cc: Thomas Gleixner <>
Fixes: dbc7f069b93a ("sched: Use replace normalize_task() with __sched_setscheduler()")
Signed-off-by: Ingo Molnar <>
Signed-off-by: Greg Kroah-Hartman <>

kernel/sched/core.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4087,8 +4087,8 @@ static int __sched_setscheduler(struct t
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) {

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