review locking to unlink lock file safely

1 files changed, 44 insertions, 6 deletions

diff --git a/kvm-wrapper.sh b/kvm-wrapper.sh
index b5ab56c..e60609d 100755
--- a/kvm-wrapper.sh
+++ b/kvm-wrapper.sh
@@ -176,6 +176,46 @@ function random_mac ()
 		|| echo -n $MACADDR
 }
 
+# stores FD in global variable LOCKFD_${VARIANT}
+# the lock will automatically be released when shell exits,
+# but calling unlock lets us unlink lock file safely
+function lock ()
+{
+	local VARIANT="$1"
+	local LOCKFILE="/tmp/kvm-wrapper-$VARIANT.lock"
+	local LOCKFD
+
+	while :; do
+		exec {LOCKFD}<>"$LOCKFILE"
+		flock ${LOCKFD}
+
+		# Check if file has changed since opening
+		# if it has, drop lock and try again
+		FD_INO=$(stat -c "%i" -L "/proc/$$/fd/${LOCKFD}")
+		FILE_INO=$(stat -c "%i" "${LOCKFILE}" 2>/dev/null)
+
+		[[ "$FD_INO" == "$FILE_INO" ]] && break
+
+		exec {LOCKFD}<&-
+	done
+
+	eval "export LOCKFD_${VARIANT}=${LOCKFD}"
+}
+
+function unlock ()
+{
+	local VARIANT="$1"
+	local LOCKFILE="/tmp/kvm-wrapper-$VARIANT.lock"
+	local LOCKFD
+
+	# remove lockfile then close (unlock) fd
+	rm -f "$LOCKFILE"
+
+	eval "LOCKFD=\$LOCKFD_${VARIANT}"
+	eval "exec $LOCKFD<&-"
+	eval "unset LOCKFD_${VARIANT}"
+}
+
 # cluster helpers
 hash_string ()
 {
@@ -474,8 +514,7 @@ function sriov()
 	[[ "$DEVDIR" = "inval" ]] && fail_exit "could not find device for $BR"
 
 	# following is racy, lock through.
-	exec {FLOCK}<>/tmp/kvm-wrapper-$BR.lock
-	flock ${FLOCK}
+	lock "$BR"
 
 	if [[ -e "$DEVDIR/sriov_numvfs" && ! -e "$DEVDIR/virtfn0" ]]; then
 		# arbitrary number: create all vfs available
@@ -488,18 +527,17 @@ function sriov()
 		[[ -e /sys/bus/pci/drivers/vfio-pci/$PCIDOMAIN ]] || break
 		PCIDOMAIN=inval
 	done
-	[[ $PCIDOMAIN = inval ]] && fail_exit "no vfio virtfn available for $BR"
+	[[ $PCIDOMAIN = inval ]] && { unlock "$BR"; fail_exit "no vfio virtfn available for $BR"; }
 	VFNUM=${VIRTFN##*virtfn}
 
 	# hack - register everything to pci-stub first so we can tell appart which are used or not
 	local PCIVENDOR="$(cat "/sys/bus/pci/devices/$PCIDOMAIN/vendor" |sed 's/^0x//')"
 	PCIVENDOR+=" $(cat "/sys/bus/pci/devices/$PCIDOMAIN/device" |sed 's/^0x//')"
-	echo "$PCIVENDOR" > "/sys/bus/pci/drivers/pci-stub/new_id" || fail_exit "couldn't add new_id (pci-stub)"
+	echo "$PCIVENDOR" > "/sys/bus/pci/drivers/pci-stub/new_id" || { unlock "$BR"; fail_exit "couldn't add new_id (pci-stub)"; }
 
 	pci_vfiofy "$PCIDOMAIN"
 
-	# unlock, can't unlink file though..
-	exec {FLOCK}<&-
+	unlock "$BR"
 }
 
 # Change perms. Meant to run forked.