This is from a bug report and conversation on IRC where Soren reported
that while a filter update is occurring on one or more VMs (due to a
rule having been edited for example), a deadlock can occur when a VM
referencing a filter is started.
The problem is caused by the two locking sequences of
qemu driver, qemu domain, filter # for the VM start operation
filter, qemu_driver, qemu_domain # for the filter update
operation
that obviously don't lock in the same order. The problem is the 2nd lock
sequence. Here the qemu_driver lock is being grabbed in
qemu_driver:qemudVMFilterRebuild()
The following solution is based on the idea of trying to re-arrange the
2nd sequence of locks as follows:
qemu_driver, filter, qemu_driver, qemu_domain
and making the qemu driver recursively lockable so that a second lock
can occur, this would then lead to the following net-locking sequence
qemu_driver, filter, qemu_domain
where the 2nd qemu_driver lock has been ( logically ) eliminated.
The 2nd part of the idea is that the sequence of locks (filter,
qemu_domain) and (qemu_domain, filter) becomes interchangeable if all
code paths where filter AND qemu_domain are locked have a preceding
qemu_domain lock that basically blocks their concurrent execution
So, the following code paths exist towards
qemu_driver:qemudVMFilterRebuild where we now want to put a qemu_driver
lock in front of the filter lock.
-> nwfilterUndefine() [ locks the filter ]
-> virNWFilterTestUnassignDef()
-> virNWFilterTriggerVMFilterRebuild()
-> qemudVMFilterRebuild()
-> nwfilterDefine()
-> virNWFilterPoolAssignDef() [ locks the filter ]
-> virNWFilterTriggerVMFilterRebuild()
-> qemudVMFilterRebuild()
-> nwfilterDriverReload()
-> virNWFilterPoolLoadAllConfigs()
->virNWFilterPoolObjLoad()
-> virNWFilterPoolAssignDef() [ locks the filter ]
-> virNWFilterTriggerVMFilterRebuild()
-> qemudVMFilterRebuild()
-> nwfilterDriverStartup()
-> virNWFilterPoolLoadAllConfigs()
->virNWFilterPoolObjLoad()
-> virNWFilterPoolAssignDef() [ locks the filter ]
-> virNWFilterTriggerVMFilterRebuild()
-> qemudVMFilterRebuild()
Qemu is not the only driver using the nwfilter driver, but also the UML
driver calls into it. Therefore qemuVMFilterRebuild() can be exchanged
with umlVMFilterRebuild() along with the driver lock of qemu_driver that
can now be a uml_driver. Further, since UML and Qemu domains can be
running on the same machine, the triggering of a rebuild of the filter
can touch both types of drivers and their domains.
In the patch below I am now extending each nwfilter callback driver with
functions for locking and unlocking the (VM) driver (UML, QEMU) and
introduce new functions for locking all registered callback drivers and
unlocking them. Then I am distributing the
lock-all-cbdrivers/unlock-all-cbdrivers call into the above call paths.
The last shown callpath starting with nwfilterDriverStart() is
problematic since it is initialize before the Qemu and UML drives are
and thus a lock in the path would result in a NULL pointer attempted to
be locked -- the call to virNWFilterTriggerVMFilterRebuild() is never
called, so we never lock either the qemu_driver or the uml_driver in
that path. Therefore, only the first 3 paths now receive calls to lock
and unlock all callback drivers. Now that the locks are distributed
where it matters I can remove the qemu_driver and uml_driver lock from
qemudVMFilterRebuild() and umlVMFilterRebuild() and not requiring the
recursive locks.
For now I want to put this out as an RFC patch. I have tested it by
'stretching' the critical section after the define/undefine functions
each lock the filter so I can (easily) concurrently execute another VM
operation (suspend,start). That code is in this patch and if you want
you can de-activate it. It seems to work ok and operations are being
blocked while the update is being done.
I still also want to verify the other assumption above that locking
filter and qemu_domain always has a preceding qemu_driver lock.
Signed-off-by: Stefan Berger <stef...@us.ibm.com>
---
src/conf/nwfilter_conf.c | 18 ++++++++++++++++++
src/conf/nwfilter_conf.h | 6 ++++++
src/libvirt_private.syms | 2 ++
src/nwfilter/nwfilter_driver.c | 20 ++++++++++++++++++++
src/qemu/qemu_driver.c | 15 +++++++++++++++
src/uml/uml_driver.c | 14 ++++++++++++++
6 files changed, 75 insertions(+)
Index: libvirt-acl/src/conf/nwfilter_conf.h
===================================================================
--- libvirt-acl.orig/src/conf/nwfilter_conf.h
+++ libvirt-acl/src/conf/nwfilter_conf.h
@@ -639,6 +639,8 @@ void virNWFilterConfLayerShutdown(void);
typedef int (*virNWFilterRebuild)(virConnectPtr conn,
virHashIterator, void *data);
+typedef void (*virNWFilterVoidCall)(void);
+
typedef struct _virNWFilterCallbackDriver virNWFilterCallbackDriver;
typedef virNWFilterCallbackDriver *virNWFilterCallbackDriverPtr;
@@ -646,9 +648,13 @@ struct _virNWFilterCallbackDriver {
const char *name;
virNWFilterRebuild vmFilterRebuild;
+ virNWFilterVoidCall vmDriverLock;
+ virNWFilterVoidCall vmDriverUnlock;
};
void virNWFilterRegisterCallbackDriver(virNWFilterCallbackDriverPtr);
+void virNWFilterCallbackDriversLock(void);
+void virNWFilterCallbackDriversUnlock(void);
VIR_ENUM_DECL(virNWFilterRuleAction);
Index: libvirt-acl/src/qemu/qemu_driver.c
===================================================================
--- libvirt-acl.orig/src/qemu/qemu_driver.c
+++ libvirt-acl/src/qemu/qemu_driver.c
@@ -12757,9 +12757,24 @@ qemudVMFiltersInstantiate(virConnectPtr
return err;
}
+
+static void
+qemudVMDriverLock(void) {
+ qemuDriverLock(qemu_driver);
+};
+
+
+static void
+qemudVMDriverUnlock(void) {
+ qemuDriverUnlock(qemu_driver);
+};
+
+
static virNWFilterCallbackDriver qemuCallbackDriver = {
.name = "QEMU",
.vmFilterRebuild = qemudVMFilterRebuild,
+ .vmDriverLock = qemudVMDriverLock,
+ .vmDriverUnlock = qemudVMDriverUnlock,
};
int qemuRegister(void) {
Index: libvirt-acl/src/uml/uml_driver.c
===================================================================
--- libvirt-acl.orig/src/uml/uml_driver.c
+++ libvirt-acl/src/uml/uml_driver.c
@@ -2219,9 +2219,23 @@ static virStateDriver umlStateDriver = {
.active = umlActive,
};
+static void
+umlVMDriverLock(void)
+{
+ umlDriverLock(uml_driver);
+}
+
+static void
+umlVMDriverUnlock(void)
+{
+ umlDriverUnlock(uml_driver);
+}
+
static virNWFilterCallbackDriver umlCallbackDriver = {
.name = "UML",
.vmFilterRebuild = umlVMFilterRebuild,
+ .vmDriverLock = umlVMDriverLock,
+ .vmDriverUnlock = umlVMDriverUnlock,
};
int umlRegister(void) {
Index: libvirt-acl/src/conf/nwfilter_conf.c
===================================================================
--- libvirt-acl.orig/src/conf/nwfilter_conf.c
+++ libvirt-acl/src/conf/nwfilter_conf.c
@@ -2161,6 +2161,24 @@ virNWFilterRegisterCallbackDriver(virNWF
}
}
+void
+virNWFilterCallbackDriversLock(void)
+{
+ int i;
+
+ for (i = 0; i < nCallbackDriver; i++)
+ callbackDrvArray[i]->vmDriverLock();
+}
+
+void
+virNWFilterCallbackDriversUnlock(void)
+{
+ int i;
+
+ for (i = 0; i < nCallbackDriver; i++)
+ callbackDrvArray[i]->vmDriverUnlock();
+}
+
static virHashIterator virNWFilterDomainFWUpdateCB;
Index: libvirt-acl/src/libvirt_private.syms
===================================================================
--- libvirt-acl.orig/src/libvirt_private.syms
+++ libvirt-acl/src/libvirt_private.syms
@@ -534,6 +534,8 @@ virNWFilterConfLayerInit;
virNWFilterConfLayerShutdown;
virNWFilterLockFilterUpdates;
virNWFilterUnlockFilterUpdates;
+virNWFilterCallbackDriversLock;
+virNWFilterCallbackDriversUnlock;
# nwfilter_params.h
Index: libvirt-acl/src/nwfilter/nwfilter_driver.c
===================================================================
--- libvirt-acl.orig/src/nwfilter/nwfilter_driver.c
+++ libvirt-acl/src/nwfilter/nwfilter_driver.c
@@ -34,6 +34,7 @@
#include "memory.h"
#include "domain_conf.h"
#include "domain_nwfilter.h"
+#include "nwfilter_conf.h"
#include "nwfilter_driver.h"
#include "nwfilter_gentech_driver.h"
@@ -152,9 +153,13 @@ nwfilterDriverReload(void) {
virNWFilterLearnThreadsTerminate(true);
nwfilterDriverLock(driverState);
+ virNWFilterCallbackDriversLock();
+
virNWFilterPoolLoadAllConfigs(conn,
&driverState->pools,
driverState->configDir);
+
+ virNWFilterCallbackDriversUnlock();
nwfilterDriverUnlock(driverState);
virConnectClose(conn);
@@ -328,12 +333,21 @@ nwfilterDefine(virConnectPtr conn,
virNWFilterPtr ret = NULL;
nwfilterDriverLock(driver);
+ virNWFilterCallbackDriversLock();
+
if (!(def = virNWFilterDefParseString(conn, xml)))
goto cleanup;
if (!(pool = virNWFilterPoolObjAssignDef(conn, &driver->pools, def)))
goto cleanup;
+#define CRITICAL_SECTION_STRETCH 10
+if (true) {
+ fprintf(stderr,"sleep in %s\n", __FUNCTION__);
+ sleep(CRITICAL_SECTION_STRETCH);
+ fprintf(stderr,"continue in %s\n", __FUNCTION__);
+}
+
if (virNWFilterPoolObjSaveDef(driver, pool, def) < 0) {
virNWFilterPoolObjRemove(&driver->pools, pool);
def = NULL;
@@ -347,6 +361,8 @@ cleanup:
virNWFilterDefFree(def);
if (pool)
virNWFilterPoolObjUnlock(pool);
+
+ virNWFilterCallbackDriversUnlock();
nwfilterDriverUnlock(driver);
return ret;
}
@@ -359,6 +375,8 @@ nwfilterUndefine(virNWFilterPtr obj) {
int ret = -1;
nwfilterDriverLock(driver);
+ virNWFilterCallbackDriversLock();
+
pool = virNWFilterPoolObjFindByUUID(&driver->pools, obj->uuid);
if (!pool) {
virNWFilterReportError(VIR_ERR_INVALID_NWFILTER,
@@ -366,6 +384,12 @@ nwfilterUndefine(virNWFilterPtr obj) {
goto cleanup;
}
+if (true) {
+ fprintf(stderr,"sleep in %s\n", __FUNCTION__);
+ sleep(CRITICAL_SECTION_STRETCH);
+ fprintf(stderr,"continue in %s\n", __FUNCTION__);
+}
+
if (virNWFilterTestUnassignDef(obj->conn, pool)) {
virNWFilterReportError(VIR_ERR_INVALID_NWFILTER,
"%s",
@@ -385,6 +409,8 @@ nwfilterUndefine(virNWFilterPtr obj) {
cleanup:
if (pool)
virNWFilterPoolObjUnlock(pool);
+
+ virNWFilterCallbackDriversUnlock();
nwfilterDriverUnlock(driver);
return ret;
}
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