Re: [PATCH] sched_ext: serialize concurrent cpu.max writers in scx_group_set_bandwidth()

On Fri, Aug 21, 2026 at 07:35:19PM +0900, Changwoo Min wrote:
> Concurrent writes to a cgroup's cpu.max are not serialized by the cgroup or
> kernfs layer -- cgroup_file_write() calls cft->write without cgroup_mutex, and
> kernfs only serializes per open file -- so two writers to the same cgroup
> through separate open files can reach tg_set_bandwidth() concurrently.
> 
> tg_set_cfs_bandwidth() serializes the CFS side under cfs_constraints_mutex,
> but scx_group_set_bandwidth() runs afterwards with only
> percpu_down_read(&scx_cgroup_ops_rwsem) held, a read lock, so it does not
> serialize concurrent writers.
> 
> The ops.cgroup_set_bandwidth() callback and the cached tg->scx.bw_* stores can
> then interleave between writers:
> 
>   CPU1 (writer A)                 CPU2 (writer B)
>   scx_group_set_bandwidth()
>     SCX_CALL_OP(...)  /* A */
>                                   scx_group_set_bandwidth()
>                                     SCX_CALL_OP(...)  /* B */
>                                     tg->scx.bw_* = B
>     tg->scx.bw_* = A
> 
> The scheduler's cgroup_set_bandwidth() op is invoked out of order and the cached
> state is left inconsistent with the last writer; the 64-bit bw_* stores can also
> tear on 32-bit.
> 
> Serialize the SCX-side update with a new scx_cgroup_set_bw_mutex held across the
> callback and the stores, so each writer applies its update atomically and in one
> order -- the SCX counterpart to cfs_constraints_mutex on the CFS side.
> 
> Reported-by: Sashiko <[email protected]>
> Link: https://lore.kernel.org/sched-ext/[email protected]/
> Signed-off-by: Changwoo Min <[email protected]>

I think the following is a better way to resolve this:

  http://lkml.kernel.org/r/[email protected]

Thanks.

-- 
tejun