block: Fix sys_ioprio_set(.which=IOPRIO_WHO_PGRP) task iteration

do_each_pid_thread() { } while_each_pid_thread() is a double loop and
thus break doesn't work as expected. Also, it should be used under
tasklist_lock because otherwise we can race against change_pid() for
PGID/SID.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/YG7Q5C4Rb5dx5GFx@hirez.programming.kicks-ass.netSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/ioprio.c

@@ -119,11 +119,17 @@ SYSCALL_DEFINE3(ioprio_set, int, which, int, who, int, ioprio)
 				pgrp = task_pgrp(current);
 			else
 				pgrp = find_vpid(who);
+
+			read_lock(&tasklist_lock);
 			do_each_pid_thread(pgrp, PIDTYPE_PGID, p) {
 				ret = set_task_ioprio(p, ioprio);
-				if (ret)
-					break;
+				if (ret) {
+					read_unlock(&tasklist_lock);
+					goto out;
+				}
 			} while_each_pid_thread(pgrp, PIDTYPE_PGID, p);
+			read_unlock(&tasklist_lock);
+
 			break;
 		case IOPRIO_WHO_USER:
 			uid = make_kuid(current_user_ns(), who);
@@ -153,6 +159,7 @@ SYSCALL_DEFINE3(ioprio_set, int, which, int, who, int, ioprio)
 			ret = -EINVAL;
 	}
 
+out:
 	rcu_read_unlock();
 	return ret;
 }