lkml.org 
[lkml]   [2020]   [Jul]   [24]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v7 3/3] sched/uclamp: Fix a deadlock when enabling uclamp static key
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);

\
 
 \ /
  Last update: 2020-07-24 12:51    [W:0.054 / U:0.700 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site