Re: [PATCH v7 3/3] sched/uclamp: Fix a deadlock when enabling uclamp static key

[Qais Yousef <qais.yousef@arm.com>]

On 07/24/20 12:41, Peter Zijlstra wrote:
> On Fri, Jul 24, 2020 at 10:46:50AM +0100, Qais Yousef wrote:
> > On 07/24/20 11:12, Peter Zijlstra wrote:
> > > On Thu, Jul 16, 2020 at 12:03:47PM +0100, Qais Yousef wrote:
> > > 
> > > I've trimmed the Changelog to read like:
> > 
> > +1
> > 
> > Should we mention the ordering issue too? Or maybe I misinterpreted the
> > 'Possible unsafe locking scenario' part?
> 
> The lock inversion was, imo, secondary. It only existed because of the
> impossible lock ordering -- taking a blocking lock inside an atomic
> lock. Fixing the first, avoids the second etc.. So I left it out.
> 
> > This should work, but you'll need to sprinkle ifdef around the key. Or move it
> > to uclamp_validate()
> 
> Indeed, the patch now reads like:

Maybe s/deadlock/splat/ in the subject now? I clearly focused on the secondary
thing..

Sorry you had to modify the patch that much yourself.

Thanks!

--
Qais Yousef

> 
> ---
> Subject: sched/uclamp: Fix a deadlock when enabling uclamp static key
> From: Qais Yousef <qais.yousef@arm.com>
> Date: Thu, 16 Jul 2020 12:03:47 +0100
> 
> From: Qais Yousef <qais.yousef@arm.com>
> 
> The following splat was caught when setting uclamp value of a task:
> 
>   BUG: sleeping function called from invalid context at ./include/linux/percpu-rwsem.h:49
> 
>    cpus_read_lock+0x68/0x130
>    static_key_enable+0x1c/0x38
>    __sched_setscheduler+0x900/0xad8
> 
> Fix by ensuring we enable the key outside of the critical section in
> __sched_setscheduler()
> 
> Fixes: 46609ce22703 ("sched/uclamp: Protect uclamp fast path code with static key")
> Signed-off-by: Qais Yousef <qais.yousef@arm.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Link: https://lkml.kernel.org/r/20200716110347.19553-4-qais.yousef@arm.com
> ---
>  kernel/sched/core.c |   11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1252,6 +1252,15 @@ static int uclamp_validate(struct task_s
>  	if (upper_bound > SCHED_CAPACITY_SCALE)
>  		return -EINVAL;
>  
> +	/*
> +	 * We have valid uclamp attributes; make sure uclamp is enabled.
> +	 *
> +	 * We need to do that here, because enabling static branches is a
> +	 * blocking operation which obviously cannot be done while holding
> +	 * scheduler locks.
> +	 */
> +	static_branch_enable(&sched_uclamp_used);
> +
>  	return 0;
>  }
>  
> @@ -1282,8 +1291,6 @@ static void __setscheduler_uclamp(struct
>  	if (likely(!(attr->sched_flags & SCHED_FLAG_UTIL_CLAMP)))
>  		return;
>  
> -	static_branch_enable(&sched_uclamp_used);
> -
>  	if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MIN) {
>  		uclamp_se_set(&p->uclamp_req[UCLAMP_MIN],
>  			      attr->sched_util_min, true);