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: --- 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);
