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

[Changwoo Min]

Hello,

On 8/22/26 4:13 AM, Tejun Heo wrote:
> 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 <sashiko-bot@xxxxxxxxxx>
> > Link: https://lore.kernel.org/sched-ext/20260817172131.BCDA51F000E9@xxxxxxxxxxxxxxx/
> > Signed-off-by: Changwoo Min <changwoo@xxxxxxxxxx>
>
> I think the following is a better way to resolve this:
>
>    http://lkml.kernel.org/r/20260821140818.1559100-1-michalblk@xxxxxxxxxx

Thanks for pointing this out. I missed the Michal's patch. Will take a look.

Regards,
Changwoo Min