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_task_acquire_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_tas_acquire_remote_curr(cpu);
if (!p)
	return;
/* ... Do something with p */
bpf_task_release(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/

v2:
https://lore.kernel.org/lkml/20250804112743.711816-1-christian.loehle@arm.com/
- Open-code bpf_task_acquire() to avoid the forward declaration (Andrea)
- Rename scx_bpf_task_acquire_remote_curr() to make it more explicit it
behaves like bpf_task_acquire()
- Dis
v1:
https://lore.kernel.org/lkml/20250801141741.355059-1-christian.loehle@arm.com/
- scx_bpf_cpu_rq() now errors when a not locked rq is requested. (Andrea)
- scx_bpf_remote_curr() calls bpf_task_acquire() which BPF user needs to
release. (Andrea)

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

 kernel/sched/ext.c                       | 38 ++++++++++++++++++++++--
 tools/sched_ext/include/scx/common.bpf.h |  1 +
 2 files changed, 37 insertions(+), 2 deletions(-)

-- 
2.34.1

On Tue, Aug 05, 2025 at 12:10:33PM +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_task_acquire_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_tas_acquire_remote_curr(cpu);
> if (!p)
> 	return;
> /* ... Do something with p */
> bpf_task_release(p);

Looks good to me.

Reviewed-by: Andrea Righi <arighi@nvidia.com>

Thanks,
-Andrea