Re: [PATCH] sched/schedutil: Fix deadlock between cpuset and cpu hotplug when using schedutil

From: Tejun Heo
Date: Tue Jul 12 2022 - 12:13:56 EST

On Tue, Jul 12, 2022 at 01:57:02PM +0100, Qais Yousef wrote:
> Is there a lot of subsystems beside cpuset that needs the cpus_read_lock()?
> A quick grep tells me it's the only one.
> Can't we instead use cpus_read_trylock() in cpuset_can_attach() so that we
> either hold the lock successfully then before we go ahead and call
> cpuset_attach(), or bail out and cancel the whole attach operation which should
> unlock the threadgroup_rwsem() lock?

But now we're failing user-initiated operations randomly. I have a hard time
seeing that as an acceptable solution. The only thing we can do, I think, is
establishing a locking order between the two locks by either nesting
threadgroup_rwsem under cpus_read_lock or disallowing thread creation during
hotplug operations.