[libvirt] [PATCH v2 4/5] qemu_hotplug: Fix a rare race condition when detaching a device twice

[Michal Privoznik]

https://bugzilla.redhat.com/show_bug.cgi?id=1623389

If a device is detached twice from the same domain the following
race condition may happen:

1) The first DetachDevice() call will issue "device_del" on qemu
monitor, but since the DEVICE_DELETED event did not arrive in
time, the API ends claiming "Device detach request sent
successfully".

2) The second DetachDevice() therefore still find the device in
the domain and thus proceeds to detaching it again. It calls
EnterMonitor() and qemuMonitorSend() trying to issue "device_del"
command again. This gets both domain lock and monitor lock
released.

3) At this point, qemu sends us the DEVICE_DELETED event which is
going to be handled by the event loop which ends up calling
qemuDomainSignalDeviceRemoval() to determine who is going to
remove the device from domain definition. Whether it is the
caller that marked the device for removal or whether it is going
to be the event processing thread.

4) Because the device was marked for removal,
qemuDomainSignalDeviceRemoval() returns true, which means the
event is to be processed by the thread that has marked the device
for removal (and is currently still trying to issue "device_del"
command)

5) The thread finally issues the "device_del" command, which
fails (obviously) and therefore it calls
qemuDomainResetDeviceRemoval() to reset the device marking and
quits immediately after, NOT removing any device from the domain
definition.

At this point, the device is still present in the domain
definition but doesn't exist in qemu anymore. Worse, there is no
way to remove it from the domain definition.

Solution is to note down that we've seen the event and if the
second "device_del" fails, not take it as a failure but carry on
with the usual execution.

Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
---
 src/qemu/qemu_domain.h  |  1 +
 src/qemu/qemu_hotplug.c | 52 +++++++++++++++++++++++++++++++++++++++--
 2 files changed, 51 insertions(+), 2 deletions(-)

diff --git a/src/qemu/qemu_domain.h b/src/qemu/qemu_domain.h
index 9f468e5661..fb361515ba 100644
--- a/src/qemu/qemu_domain.h
+++ b/src/qemu/qemu_domain.h
@@ -218,6 +218,7 @@ typedef qemuDomainUnpluggingDevice *qemuDomainUnpluggingDevicePtr;
 struct _qemuDomainUnpluggingDevice {
     const char *alias;
     qemuDomainUnpluggingDeviceStatus status;
+    bool eventSeen; /* True if DEVICE_DELETED event arrived. */
 };
 
 
diff --git a/src/qemu/qemu_hotplug.c b/src/qemu/qemu_hotplug.c
index 59a5d21a5b..6b5b37bda5 100644
--- a/src/qemu/qemu_hotplug.c
+++ b/src/qemu/qemu_hotplug.c
@@ -67,6 +67,9 @@ VIR_LOG_INIT("qemu.qemu_hotplug");
 unsigned long long qemuDomainRemoveDeviceWaitTime = 1000ull * 5;
 
 
+static void
+qemuDomainResetDeviceRemoval(virDomainObjPtr vm);
+
 /**
  * qemuDomainDeleteDevice:
  * @vm: domain object
@@ -103,8 +106,51 @@ qemuDomainDeleteDevice(virDomainObjPtr vm,
     }
 
     if (enterMonitor &&
-        qemuDomainObjExitMonitor(driver, vm) < 0)
-        rc = -1;
+        qemuDomainObjExitMonitor(driver, vm) < 0) {
+        /* Domain is no longer running. No cleanup needed. */
+        return -1;
+    }
+
+    if (rc < 0) {
+        bool eventSeen;
+
+        /* Deleting device failed. Let's check if DEVICE_DELETED
+         * even arrived. If it did, we need to claim success to
+         * make the caller remove device from domain XML. */
+        if (enterMonitor) {
+            /* Here @vm is locked again. It's safe to access
+             * private data directly. */
+        } else {
+            /* Here @vm is not locked. Do some locking magic to
+             * be able to access private data. It is safe to lock
+             * and unlock both @mon and @vm here because:
+             * a) qemuDomainObjEnterMonitor() ensures @mon is ref()'d
+             * b) The API that is calling us ensures that @vm is ref()'d
+             */
+            virObjectUnlock(priv->mon);
+            virObjectLock(vm);
+            virObjectLock(priv->mon);
+        }
+
+        eventSeen = priv->unplug.eventSeen;
+        if (eventSeen) {
+            /* The event arrived. Return success. */
+            VIR_DEBUG("Detaching of device %s failed, but event arrived", alias);
+            qemuDomainResetDeviceRemoval(vm);
+            rc = 0;
+        } else if (rc == -2) {
+            /* The device does not exist in qemu, but it still
+             * exists in libvirt. Claim success to make caller
+             * qemuDomainWaitForDeviceRemoval(). Otherwise if
+             * domain XML is queried right after detach API the
+             * device would still be there.  */
+            VIR_DEBUG("Detaching of device %s failed and no event arrived", alias);
+            rc = 0;
+        }
+
+        if (!enterMonitor)
+            virObjectUnlock(vm);
+    }
 
     return rc;
 }
@@ -5186,6 +5232,7 @@ qemuDomainResetDeviceRemoval(virDomainObjPtr vm)
 {
     qemuDomainObjPrivatePtr priv = vm->privateData;
     priv->unplug.alias = NULL;
+    priv->unplug.eventSeen = false;
 }
 
 /* Returns:
@@ -5245,6 +5292,7 @@ qemuDomainSignalDeviceRemoval(virDomainObjPtr vm,
         VIR_DEBUG("Removal of device '%s' continues in waiting thread", devAlias);
         qemuDomainResetDeviceRemoval(vm);
         priv->unplug.status = status;
+        priv->unplug.eventSeen = true;
         virDomainObjBroadcast(vm);
         return true;
     }
-- 
2.19.2