[PATCH v2 04/10] sched_ext: Fix ops.cgroup_move() invocation kf_mask and rq tracking

[Tejun Heo]

sched_move_task() invokes ops.cgroup_move() inside task_rq_lock(tsk), so
@p's rq lock is held. The SCX_CALL_OP_TASK invocation mislabels this:

  - kf_mask = SCX_KF_UNLOCKED (== 0), claiming no lock is held.
  - rq = NULL, so update_locked_rq() doesn't run and scx_locked_rq()
    returns NULL.

Switch to SCX_KF_REST and pass task_rq(p), matching ops.set_cpumask()
from set_cpus_allowed_scx().

Three effects:

  - scx_bpf_task_cgroup() becomes callable (was rejected by
    scx_kf_allowed(__SCX_KF_RQ_LOCKED)). Safe; rq lock is held.

  - scx_bpf_dsq_move() is now rejected (was allowed via the unlocked
    branch). Calling it while holding an unrelated task's rq lock is
    risky; rejection is correct.

  - scx_bpf_select_cpu_*() previously took the unlocked branch in
    select_cpu_from_kfunc() and called task_rq_lock(p, &rf), which
    would deadlock against the already-held pi_lock. Now it takes the
    locked-rq branch and is rejected with -EPERM via the existing
    kf_allowed(SCX_KF_SELECT_CPU | SCX_KF_ENQUEUE) check. Latent
    deadlock fix.

No in-tree scheduler is known to call any of these from ops.cgroup_move().

v2: Add Fixes: tag (Andrea Righi).

Fixes: 18853ba782be ("sched_ext: Track currently locked rq")
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Reviewed-by: Andrea Righi <arighi@xxxxxxxxxx>
---
 kernel/sched/ext.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -4397,7 +4397,7 @@ void scx_cgroup_move_task(struct task_st
 	 */
 	if (SCX_HAS_OP(sch, cgroup_move) &&
 	    !WARN_ON_ONCE(!p->scx.cgrp_moving_from))
-		SCX_CALL_OP_TASK(sch, SCX_KF_UNLOCKED, cgroup_move, NULL,
+		SCX_CALL_OP_TASK(sch, SCX_KF_REST, cgroup_move, task_rq(p),
 				 p, p->scx.cgrp_moving_from,
 				 tg_cgrp(task_group(p)));
 	p->scx.cgrp_moving_from = NULL;