From 9e1beaad709ba68a82a54738ad7b50cac69861cc Mon Sep 17 00:00:00 2001
From: Stefan Hajnoczi <stefanha@redhat.com>
Date: Fri, 17 Nov 2017 11:19:04 +0100
Subject: [PATCH 05/15] throttle-groups: drain before detaching ThrottleState

RH-Author: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: <20171117111908.8815-6-stefanha@redhat.com>
Patchwork-id: 77740
O-Subject: [RHV7.5 qemu-kvm-rhev PATCH 5/9] throttle-groups: drain before detaching ThrottleState
Bugzilla: 1492295
RH-Acked-by: John Snow <jsnow@redhat.com>
RH-Acked-by: Laurent Vivier <lvivier@redhat.com>
RH-Acked-by: Thomas Huth <thuth@redhat.com>

I/O requests hang after stop/cont commands at least since QEMU 2.10.0
with -drive iops=100:

  (guest)$ dd if=/dev/zero of=/dev/vdb oflag=direct count=1000
  (qemu) stop
  (qemu) cont
  ...I/O is stuck...

This happens because blk_set_aio_context() detaches the ThrottleState
while requests may still be in flight:

  if (tgm->throttle_state) {
      throttle_group_detach_aio_context(tgm);
      throttle_group_attach_aio_context(tgm, new_context);
  }

This patch encloses the detach/attach calls in a drained region so no
I/O request is left hanging.  Also add assertions so we don't make the
same mistake again in the future.

Reported-by: Yongxue Hong <yhong@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-id: 20171110151934.16883-1-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit dc868fb03b9b829ed9d2ecdae0fcc12f3fe19b4f)
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
---
 block/block-backend.c   | 2 ++
 block/throttle-groups.c | 6 ++++++
 2 files changed, 8 insertions(+)

diff --git a/block/block-backend.c b/block/block-backend.c
index bfb3e84..520f2b2 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -1771,8 +1771,10 @@ void blk_set_aio_context(BlockBackend *blk, AioContext *new_context)
 
     if (bs) {
         if (tgm->throttle_state) {
+            bdrv_drained_begin(bs);
             throttle_group_detach_aio_context(tgm);
             throttle_group_attach_aio_context(tgm, new_context);
+            bdrv_drained_end(bs);
         }
         bdrv_set_aio_context(bs, new_context);
     }
diff --git a/block/throttle-groups.c b/block/throttle-groups.c
index 7749cf0..44cc1e6 100644
--- a/block/throttle-groups.c
+++ b/block/throttle-groups.c
@@ -568,6 +568,12 @@ void throttle_group_attach_aio_context(ThrottleGroupMember *tgm,
 void throttle_group_detach_aio_context(ThrottleGroupMember *tgm)
 {
     ThrottleTimers *tt = &tgm->throttle_timers;
+
+    /* Requests must have been drained */
+    assert(tgm->pending_reqs[0] == 0 && tgm->pending_reqs[1] == 0);
+    assert(qemu_co_queue_empty(&tgm->throttled_reqs[0]));
+    assert(qemu_co_queue_empty(&tgm->throttled_reqs[1]));
+
     throttle_timers_detach_aio_context(tt);
     tgm->aio_context = NULL;
 }
-- 
1.8.3.1