Re: [PATCH v3] scx: Fix raciness in scx_ops_bypass()

[Tejun Heo]

On Fri, Oct 25, 2024 at 03:54:08PM -0500, David Vernet wrote:
> scx_ops_bypass() can currently race on the ops enable / disable path as
> follows:
> 
> 1. scx_ops_bypass(true) called on enable path, bypass depth is set to 1
> 2. An op on the init path exits, which schedules scx_ops_disable_workfn()
> 3. scx_ops_bypass(false) is called on the disable path, and bypass depth
>    is decremented to 0
> 4. kthread is scheduled to execute scx_ops_disable_workfn()
> 5. scx_ops_bypass(true) called, bypass depth set to 1
> 6. scx_ops_bypass() races when iterating over CPUs
> 
> While it's not safe to take any blocking locks on the bypass path, it is
> safe to take a raw spinlock which cannot be preempted. This patch therefore
> updates scx_ops_bypass() to use a raw spinlock to synchronize, and changes
> scx_ops_bypass_depth to be a regular int.
> 
> Without this change, we observe the following warnings when running the
> 'exit' sched_ext selftest (sometimes requires a couple of runs):
...
> Fixes: f0e1a0643a59 ("sched_ext: Implement BPF extensible scheduler class")
> Signed-off-by: David Vernet <void@xxxxxxxxxxxxx>

Applied to sched_ext/for-6.12-fixes. Thanks!

-- 
tejun