scx_bpf_cpu_rq() currently allows accessing struct rq fields without
holding the associated rq.
It is being used by scx_cosmos, scx_flash, scx_lavd, scx_layered, and
scx_tickless. Fortunately it is only ever used to fetch rq->curr.
So provide an alternative scx_bpf_remote_curr() that doesn't expose
struct rq and harden scx_bpf_cpu_rq() by ensuring we hold the rq lock.

This also simplifies scx code from:

rq = scx_bpf_cpu_rq(cpu);
if (!rq)
	return;
p = rq->curr
if (!p)
	return;
/* ... Do something with p */

into:

p = scx_bpf_remote_curr(cpu);
if (!p)
	return;
/* ... Do something with p */


Patch 1 was previously submitted and can be applied independently of
the other two.
https://lore.kernel.org/lkml/43a9cbdc-5121-4dc8-8438-0f01c90a4687@arm.com/
https://lore.kernel.org/lkml/0b8111c6-1b14-41dc-a674-14a6361992b3@arm.com/

Christian Loehle (3):
  sched_ext: Mark scx_bpf_cpu_rq as NULL returnable
  sched_ext: Provide scx_bpf_remote_curr()
  sched_ext: Guarantee rq lock on scx_bpf_cpu_rq()

 kernel/sched/ext.c                       | 21 ++++++++++++++++++---
 tools/sched_ext/include/scx/common.bpf.h |  1 +
 2 files changed, 19 insertions(+), 3 deletions(-)

--
2.34.1

Hi Christian,

thanks for tackling this! Comments below.

On Fri, Aug 01, 2025 at 03:17:38PM +0100, Christian Loehle wrote:
> scx_bpf_cpu_rq() currently allows accessing struct rq fields without
> holding the associated rq.
> It is being used by scx_cosmos, scx_flash, scx_lavd, scx_layered, and
> scx_tickless. Fortunately it is only ever used to fetch rq->curr.
> So provide an alternative scx_bpf_remote_curr() that doesn't expose
> struct rq and harden scx_bpf_cpu_rq() by ensuring we hold the rq lock.
> 
> This also simplifies scx code from:
> 
> rq = scx_bpf_cpu_rq(cpu);
> if (!rq)
> 	return;
> p = rq->curr
> if (!p)
> 	return;
> /* ... Do something with p */
> 
> into:
> 
> p = scx_bpf_remote_curr(cpu);
> if (!p)
> 	return;
> /* ... Do something with p */

To be 100% correct I think we should do something similar to
bpf_task_from_pid(), acquire a reference to the task and release it via
bpf_task_release().

Basically:

 p = scx_bpf_remote_curr(cpu);
 if (!p)
 	return;
 /* ... Do something with p */
 bpf_task_release(p);

Thanks
-Andrea