Bug: https://bugs.gentoo.org/891871

--- a/common.sh
+++ b/common.sh
@@ -172,11 +172,11 @@
 	done
 }
 
-#check_btrfs_mounted $bootsv $bootuuid)
+#check_btrfs_mounted $bootsv $bootsig)
 check_btrfs_mounted () {
 	bootsv="$1"
-	bootuuid="$2"
-	bootdev=$(blkid | grep "$bootuuid" | cut -d ':' -f  1)
+	bootsig="$2"
+	bootdev=$(blkid | grep "$bootsig" | cut -d ':' -f  1)
 	bindfrom=$(grep " btrfs " /proc/self/mountinfo |
 		   grep " $bootdev " | grep " /$bootsv " | cut -d ' ' -f 5)
 	printf "%s" "$bindfrom"
--- a/linux-boot-prober
+++ b/linux-boot-prober
@@ -98,10 +98,10 @@
 		bootsv="$subvol"
 	elif echo "$bootmnt" | cut -d ' ' -f 3 | grep -q "btrfs"; then
 		# separate btrfs /boot subvolume
-		bootsv=$(echo "$bootmnt" | cut -d ' ' -f 4 | grep "^subvol=" | sed "s/subvol=//" )
-		bootuuid=$(echo "$bootmnt" | cut -d ' ' -f 1 | grep "^UUID=" | sed "s/UUID=//" )
-		debug "mounting btrfs $tmpmnt/boot UUID=$bootuuid subvol=$bootsv"
-		bindfrom=$(check_btrfs_mounted $bootsv $bootuuid)
+		bootsv=$(echo "$bootmnt" | cut -d ' ' -f 4 | grep "\(^\|,\)subvol=" | sed "s/.*subvol=\([^,]*\).*$/\1/")
+		bootsig=$(echo "$bootmnt" | cut -d ' ' -f 1 | grep "^\(PART\)\?\(UUID\|LABEL\)=" | sed 's/\(.*=\)\(.*\)/\1"\2"/' )
+		debug "mounting btrfs $tmpmnt/boot $bootsig subvol=$bootsv"
+		bindfrom=$(check_btrfs_mounted $bootsv $bootsig)
 		if [ -n "$bindfrom" ]; then
 			# already mounted some place
 			if ! mount -o bind $bindfrom "$tmpmnt/boot" 2>/dev/null; then
@@ -111,14 +111,14 @@
 				rmdir "$tmpmnt" 2>/dev/null
 				exit 1
 			fi
-		elif ! mount -o subvol=$bootsv -t btrfs -U $bootuuid "$tmpmnt/boot" 2>/dev/null; then
-			warn "error mounting btrfs boot partition subvol=$bootsv, UUID=$bootuuid"
+		elif ! mount -o subvol=$bootsv -t btrfs $bootsig "$tmpmnt/boot" 2>/dev/null; then
+			warn "error mounting btrfs boot partition subvol=$bootsv, $bootsig"
 			umount "$tmpmnt/boot" 2>/dev/null
 			umount "$tmpmnt" 2>/dev/null
 			rmdir "$tmpmnt" 2>/dev/null
 			exit 1
 		fi
-		bootpart=$(grep " btrfs " /proc/self/mountinfo | grep " /$bootsv " | cut -d ' ' -f 10)
+		bootpart=$(grep " btrfs " /proc/self/mountinfo | grep " /${bootsv##/} " | cut -d ' ' -f 10)
 	else
 		# non-btrfs partition or logical volume
 		linux_mount_boot $partition $tmpmnt
--- a/linux-boot-probes/mounted/common/40grub2
+++ b/linux-boot-probes/mounted/common/40grub2
@@ -20,7 +20,7 @@
 		kernel=${kernel#${bsv}}
 		kernelfile=$kernel
 		initrd=${initrd#${bsv}}
-		if [ "x$GRUB_FS" != "xbtrfs" ]; then
+		if [ "x$GRUB_FS" != "xbtrfs" -o "$partition" != "$bootpart" ]; then
 		    # absolute path needed: prepend subvolume if $kernel isn't empty
 		    kernel=${kernel:+${bsv}}${kernel}
 		    # handle multiple initrd paths
--- a/os-probes/common/50mounted-tests
+++ b/os-probes/common/50mounted-tests
@@ -85,9 +85,9 @@
 		exit 1
 	fi
 	# besides regular subvols, get ro and snapshot so thet can be excluded
-        subvols=$(btrfs subvolume list "$tmpmnt" | cut -d ' ' -f 9)
-        rosubvols=$(btrfs subvolume list -r "$tmpmnt" | cut -d ' ' -f 9)
-        sssubvols=$(btrfs subvolume list -s "$tmpmnt" | cut -d ' ' -f 14)
+        subvols=$(btrfs subvolume list -a "$tmpmnt" | cut -d ' ' -f 9 | sed 's/<FS_TREE>\///')
+        rosubvols=$(btrfs subvolume list -ra "$tmpmnt" | cut -d ' ' -f 9 | sed 's/<FS_TREE>\///')
+        sssubvols=$(btrfs subvolume list -sa "$tmpmnt" | cut -d ' ' -f 14 | sed 's/<FS_TREE>\///')
         if ! umount "$tmpmnt"; then
             warn "failed to umount btrfs volume on $tmpmnt"
             rmdir "$tmpmnt" || true