From a3176697f2d1288d3e30ca6aac0d2c5891a6b135 Mon Sep 17 00:00:00 2001
Message-Id: <a3176697f2d1288d3e30ca6aac0d2c5891a6b135@dist-git>
From: Peter Krempa <pkrempa@redhat.com>
Date: Thu, 26 May 2016 12:54:54 +0200
Subject: [PATCH] qemu: bulk stats: Don't access possibly blocked storage

https://bugzilla.redhat.com/show_bug.cgi?id=1339963

If the stats for a block device can't be acquired from qemu we've
fallen back to loading them from the file on the disk in libvirt.

If qemu is not cooperating due to being stuck on an inaccessible NFS
share we would then attempt to read the files and get stuck too with
the VM object locked. All other APIs would eventually get stuck waiting
on the VM lock.

Avoid this problem by skipping the block stats if the VM is online but
the monitor did not provide any stats.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1337073
(cherry picked from commit 71d2c172edb997bae1e883b2e1bafa97d9f953a1)
---
 src/qemu/qemu_driver.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c
index 1075237..5e28b21 100644
--- a/src/qemu/qemu_driver.c
+++ b/src/qemu/qemu_driver.c
@@ -19392,13 +19392,22 @@ qemuDomainGetStatsOneBlock(virQEMUDriverPtr driver,
         QEMU_ADD_BLOCK_PARAM_UI(record, maxparams, block_idx, "backingIndex",
                                 backing_idx);
 
-    /* use fallback path if data is not available */
-    if (!stats || !alias || !(entry = virHashLookup(stats, alias))) {
+    /* the VM is offline so we have to go and load the stast from the disk by
+     * ourselves */
+    if (!virDomainObjIsActive(dom)) {
         ret = qemuDomainGetStatsOneBlockFallback(driver, cfg, dom, record,
                                                  maxparams, src, block_idx);
         goto cleanup;
     }
 
+    /* In case where qemu didn't provide the stats we stop here rather than
+     * trying to refresh the stats from the disk. Inability to provide stats is
+     * usually caused by blocked storage so this would make libvirtd hang */
+    if (!stats || !alias || !(entry = virHashLookup(stats, alias))) {
+        ret = 0;
+        goto cleanup;
+    }
+
     QEMU_ADD_BLOCK_PARAM_LL(record, maxparams, block_idx,
                             "rd.reqs", entry->rd_req);
     QEMU_ADD_BLOCK_PARAM_LL(record, maxparams, block_idx,
-- 
2.8.3