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

[Steven Rostedt <rostedt@goodmis.org>]

Peter,

I've just been pinged by someone that triggered this bug again. Can you
take this patch and it probably should be marked for stable too.

-- Steve


On Thu, 9 Mar 2017 10:18:42 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:

> From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> 
> 
> 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@gandalf.local.home
> 
> Reported-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
> Tested-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
> Fixes: dbc7f069b93a ("sched: Use replace normalize_task() with __sched_setscheduler()")
> Cc: Andrew Morton <akpm@osdl.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@elte.hu>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: stable@vger.kernel.org
> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> ---
>  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());
>  recheck:
>  	/* Double check policy once rq lock held: */
>  	if (policy < 0) {