[PATCH v2 1/3] x86/intel_rdt: Check monitor group vs control group membership earlier

[Valentin Schneider]

A task can only be moved between monitor groups if both groups belong to
the same control group. This is checked fairly late however: by that time
we already have appended a task_work() callback, the execution of which
will be useless (there are no closid/rmid updates to handle, barring
concurrent writes).

Check the validity of the move earlier to save any kzalloc() /
task_work_add() if it wasn't going to be necessary.

Reviewed-by: James Morse <James.Morse@xxxxxxx>
Signed-off-by: Valentin Schneider <valentin.schneider@xxxxxxx>
---
 arch/x86/kernel/cpu/resctrl/rdtgroup.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
index af323e2e3100..b6b5b95df833 100644
--- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
+++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
@@ -581,12 +581,7 @@ static int __rdtgroup_move_task(struct task_struct *tsk,
 			tsk->closid = rdtgrp->closid;
 			tsk->rmid = rdtgrp->mon.rmid;
 		} else if (rdtgrp->type == RDTMON_GROUP) {
-			if (rdtgrp->mon.parent->closid == tsk->closid) {
-				tsk->rmid = rdtgrp->mon.rmid;
-			} else {
-				rdt_last_cmd_puts("Can't move task to different control group\n");
-				ret = -EINVAL;
-			}
+			tsk->rmid = rdtgrp->mon.rmid;
 		}
 	}
 	return ret;
@@ -673,9 +668,19 @@ static int rdtgroup_move_task(pid_t pid, struct rdtgroup *rdtgrp,
 	rcu_read_unlock();
 
 	ret = rdtgroup_task_write_permission(tsk, of);
-	if (!ret)
-		ret = __rdtgroup_move_task(tsk, rdtgrp);
+	if (ret)
+		goto out;
 
+	if (rdtgrp->type == RDTMON_GROUP &&
+	    rdtgrp->mon.parent->closid != tsk->closid) {
+		rdt_last_cmd_puts("Can't move task to different control group\n");
+		ret = -EINVAL;
+		goto out;
+	}
+
+	ret = __rdtgroup_move_task(tsk, rdtgrp);
+
+out:
 	put_task_struct(tsk);
 	return ret;
 }
-- 
2.27.0