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Re: [libvirt] [PATCH v2 2/2] qemu: avoid rare race when undefining domain

On Mon, Nov 03, 2014 at 09:54:09AM +0100, Michal Privoznik wrote:
On 31.10.2014 15:47, Martin Kletzander wrote:
When one domain is being undefined and at the same time started, for
example, there is a possibility of a rare problem occuring.

  - Thread 1 does virDomainUndefine(), has the lock, checks that the
    domain is active and because it's not, calls

  - Thread 2 does virDomainCreate() and tries to lock the domain.

  - Thread 1 needs to lock domain list in order to remove the domain from
    it, but must unlock domain first (proper order is to lock domain list
    first and the domain itself second).

  - Thread 2 grabs the lock, starts the domain and releases the lock.

  - Thread 1 grabs the lock and removes the domain from list.

With this patch:

  - qemuDomainRemoveInactive() creates a QEMU_JOB_MODIFY if that's
    possible, but since it must remove the domain from list either way,
    it continues even when starting the job failed.

  - Because virDomainObjListRemove() removes a reference to the
    virDomainObj (and it must be kept that way for other drivers), we
    hack around that by acquiring one more reference that's kept until
    the job is ended.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1150505

Signed-off-by: Martin Kletzander <mkletzan redhat com>
  src/qemu/qemu_domain.c | 9 +++++++++
  1 file changed, 9 insertions(+)

diff --git a/src/qemu/qemu_domain.c b/src/qemu/qemu_domain.c
index 76fccce..98c4763 100644
--- a/src/qemu/qemu_domain.c
+++ b/src/qemu/qemu_domain.c
@@ -2392,9 +2392,13 @@ void
  qemuDomainRemoveInactive(virQEMUDriverPtr driver,
                           virDomainObjPtr vm)
+    bool haveJob = true;
      char *snapDir;
      virQEMUDriverConfigPtr cfg = virQEMUDriverGetConfig(driver);

+    if (qemuDomainObjBeginJob(driver, vm, QEMU_JOB_MODIFY) < 0)
+        haveJob = false;
      /* Remove any snapshot metadata prior to removing the domain */
      if (qemuDomainSnapshotDiscardAllMetadata(driver, vm) < 0) {
          VIR_WARN("unable to remove all snapshots for domain %s",
@@ -2409,8 +2413,13 @@ qemuDomainRemoveInactive(virQEMUDriverPtr driver,
              VIR_WARN("unable to remove snapshot directory %s", snapDir);
+    virObjectRef(vm);
      virDomainObjListRemove(driver->domains, vm);
+    if (haveJob)
+        ignore_value(qemuDomainObjEndJob(driver, vm));
+    virObjectUnref(vm);


I don't think there's a reason to do the Ref() and Unref(). Job control
functions do that already. ACK with that removed.

Oh, they do :)  I removed those two lines and the part of commit
message as well.  And I'll push it in a while.


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