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From 1b487f2a77c49129267e4f779956dd549684d8d9 Mon Sep 17 00:00:00 2001
From: Stefan Hajnoczi <>
Date: Mon, 23 Jul 2018 16:36:29 +0200
Subject: [PATCH 256/268] throttle-groups: fix hang when group member leaves

RH-Author: Stefan Hajnoczi <>
Message-id: <>
Patchwork-id: 81469
O-Subject: [RHEL-7.6 qemu-kvm-rhev PATCH 1/1] throttle-groups: fix hang when group member leaves
Bugzilla: 1535914
RH-Acked-by: Fam Zheng <>
RH-Acked-by: Laurent Vivier <>
RH-Acked-by: Miroslav Rezanina <>

Throttle groups consist of members sharing one throttling state
(including bps/iops limits).  Round-robin scheduling is used to ensure
fairness.  If a group member already has a timer pending then other
groups members do not schedule their own timers.  The next group member
will have its turn when the existing timer expires.

A hang may occur when a group member leaves while it had a timer
scheduled.  Although the code carefully removes the group member from
the round-robin list, it does not schedule the next member.  Therefore
remaining members continue to wait for the removed member's timer to

This patch schedules the next request if a timer is pending.
Unfortunately the actual bug is a race condition that I've been unable
to capture in a test case.

Sometimes drive2 hangs when drive1 is removed from the throttling group:

  $ qemu ... -drive if=none,id=drive1,cache=none,format=qcow2,file=data1.qcow2,iops=100,group=foo \
             -device virtio-blk-pci,id=virtio-blk-pci0,drive=drive1 \
             -drive if=none,id=drive2,cache=none,format=qcow2,file=data2.qcow2,iops=10,group=foo \
             -device virtio-blk-pci,id=virtio-blk-pci1,drive=drive2
  (guest-console1)# fio -filename /dev/vda 4k-seq-read.job
  (guest-console2)# fio -filename /dev/vdb 4k-seq-read.job
  (qmp) {"execute": "block_set_io_throttle", "arguments": {"device": "drive1","bps": 0,"bps_rd": 0,"bps_wr": 0,"iops": 0,"iops_rd": 0,"iops_wr": 0}}

Reported-by: Nini Gu <>
Signed-off-by: Stefan Hajnoczi <>
Cc: Alberto Garcia <>
Signed-off-by: Stefan Hajnoczi <>
Signed-off-by: Miroslav Rezanina <>
 block/throttle-groups.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/block/throttle-groups.c b/block/throttle-groups.c
index 36cc043..e297b04 100644
--- a/block/throttle-groups.c
+++ b/block/throttle-groups.c
@@ -564,6 +564,10 @@ void throttle_group_unregister_tgm(ThrottleGroupMember *tgm)
     for (i = 0; i < 2; i++) {
+        if (timer_pending(tgm->throttle_timers.timers[i])) {
+            tg->any_timer_armed[i] = false;
+            schedule_next_request(tgm, i);
+        }
         if (tg->tokens[i] == tgm) {
             token = throttle_group_next_tgm(tgm);
             /* Take care of the case where this is the last tgm in the group */