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

[Peter Zijlstra]

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) <pet...@infradead.org>
---
 block/ioprio.c |   11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

--- a/block/ioprio.c
+++ b/block/ioprio.c
@@ -119,11 +119,17 @@ SYSCALL_DEFINE3(ioprio_set, int, which,
                                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,
                        ret = -EINVAL;
        }
 
+out:
        rcu_read_unlock();
        return ret;
 }