commit 82bb167f0d15b733b23931205be3488b83cb9ec6
Author: Michal Prívozník <mprivozn@redhat.com>
Date:   Tue Aug 18 11:08:15 2020 +0200

    virdevmapper: Don't cache device-mapper major
    
    The device mapper major is needed in virIsDevMapperDevice() which
    determines whether given device is managed by device-mapper. This
    number is obtained by parsing /proc/devices and then stored in a
    global variable so that the file doesn't have to be parsed again.
    However, as it turns out this logic is flawed - the major number
    is not static and can change as it can be specified as a
    parameter when loading the dm-mod module.
    
    Unfortunately, I was not able to come up with a good solution and
    thus the /proc/devices file is being parsed every time we need
    the device mapper major.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
    Reviewed-by: Peter Krempa <pkrempa@redhat.com>
    Reviewed-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>
    Tested-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>

Index: libvirt-6.6.0/src/util/virdevmapper.c
===================================================================
--- libvirt-6.6.0.orig/src/util/virdevmapper.c
+++ libvirt-6.6.0/src/util/virdevmapper.c
@@ -46,11 +46,9 @@
 
 G_STATIC_ASSERT(BUF_SIZE > sizeof(struct dm_ioctl));
 
-static unsigned int virDMMajor;
-
 
 static int
-virDevMapperOnceInit(void)
+virDevMapperGetMajor(unsigned int *major)
 {
     g_autofree char *buf = NULL;
     VIR_AUTOSTRINGLIST lines = NULL;
@@ -69,7 +67,7 @@ virDevMapperOnceInit(void)
 
         if (sscanf(lines[i], "%u %ms\n", &maj, &dev) == 2 &&
             STREQ(dev, DM_NAME)) {
-            virDMMajor = maj;
+            *major = maj;
             break;
         }
     }
@@ -85,9 +83,6 @@ virDevMapperOnceInit(void)
 }
 
 
-VIR_ONCE_GLOBAL_INIT(virDevMapper);
-
-
 static void *
 virDMIoctl(int controlFD, int cmd, struct dm_ioctl *dm, char **buf)
 {
@@ -305,9 +300,6 @@ virDevMapperGetTargets(const char *path,
      * consist of devices or yet another targets. If that's the
      * case, we have to stop recursion somewhere. */
 
-    if (virDevMapperInitialize() < 0)
-        return -1;
-
     if ((controlFD = virDMOpen()) < 0)
         return -1;
 
@@ -319,13 +311,14 @@ bool
 virIsDevMapperDevice(const char *dev_name)
 {
     struct stat buf;
+    unsigned int major;
 
-    if (virDevMapperInitialize() < 0)
+    if (virDevMapperGetMajor(&major) < 0)
         return false;
 
     if (!stat(dev_name, &buf) &&
         S_ISBLK(buf.st_mode) &&
-        major(buf.st_rdev) == virDMMajor)
+        major(buf.st_rdev) == major)
         return true;
 
     return false;