4.4-stable review patch. If anyone has any objections, please let me know.
------------------ From: Steven Rostedt (VMware) <[email protected]> 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 <[email protected]> Tested-by: Laurent Dufour <[email protected]> Signed-off-by: Steven Rostedt (VMware) <[email protected]> Signed-off-by: Peter Zijlstra (Intel) <[email protected]> Cc: Andrew Morton <[email protected]> Cc: Linus Torvalds <[email protected]> Cc: Peter Zijlstra <[email protected]> Cc: Thomas Gleixner <[email protected]> Fixes: dbc7f069b93a ("sched: Use replace normalize_task() with __sched_setscheduler()") Link: http://lkml.kernel.org/r/[email protected] Signed-off-by: Ingo Molnar <[email protected]> Signed-off-by: Greg Kroah-Hartman <[email protected]> --- kernel/sched/core.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -3822,8 +3822,8 @@ static int __sched_setscheduler(struct t struct rq *rq; int reset_on_fork; - /* may grab non-irq protected spin_locks */ - BUG_ON(in_interrupt()); + /* The pi code expects interrupts enabled */ + BUG_ON(pi && in_interrupt()); recheck: /* double check policy once rq lock held */ if (policy < 0) {
