Re: [PATCHSET sched_ext/for-6.12-fixes] sched_ext: Fix locking enable/disable path bugs includling locking order one
From: Tejun Heo
Date: Fri Sep 27 2024 - 16:03:58 EST
On Mon, Sep 23, 2024 at 08:59:27AM -1000, Tejun Heo wrote:
> Aboorva Devarajan reported an issue where sched_ext init code can
> occasionally deadlock when scheduler loading races CPU hotplug. The deadlock
> scenario is as follows:
>
> scx_ops_enable() hotplug
>
> percpu_down_write(&cpu_hotplug_lock)
> percpu_down_write(&scx_fork_rwsem)
> block on cpu_hotplug_lock
> kthread_create() waits for kthreadd
> kthreadd blocks on scx_fork_rwsem
>
> Note that this doesn't trigger lockdep because the hotplug side dependency
> bounces through kthreadd.
>
> This is primarily caused by SCX enable/disable paths grabbing big locks
> together. This patchset updates the enable/disable paths to decouple the
> locks. In the process, it also fixes several subtle bugs in the enable path.
>
> This patchset contains the following patches:
>
> 0001-sched_ext-Relocate-check_hotplug_seq-call-in-scx_ops.patch
> 0002-sched_ext-Remove-SCX_OPS_PREPPING.patch
> 0003-sched_ext-Initialize-in-bypass-mode.patch
> 0004-sched_ext-Fix-SCX_TASK_INIT-SCX_TASK_READY-transitio.patch
> 0005-sched_ext-Enable-scx_ops_init_task-separately.patch
> 0006-sched_ext-Add-scx_cgroup_enabled-to-gate-cgroup-oper.patch
> 0007-sched_ext-Decouple-locks-in-scx_ops_disable_workfn.patch
> 0008-sched_ext-Decouple-locks-in-scx_ops_enable.patch
Applied to sched_ext/for-6.12-fixes.
Thanks.
--
tejun