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 <[email protected]> > Date: Thu, 16 Jul 2020 12:03:47 +0100 > > From: Qais Yousef <[email protected]> > > 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 <[email protected]> > Signed-off-by: Peter Zijlstra (Intel) <[email protected]> > Link: https://lkml.kernel.org/r/[email protected] > --- > 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);
