loop-util: rework how we lock loopback block devices

Let's rework how we lock loopback block devices in two ways:

1. Lock a separate fd, instead of the main block device fd. We already
   did that for our internal locking when allocating loopback block
   devices, but do so for the exposed locking (i.e.
   loop_device_flock()), too, so that the lock is independent of the
   main fd we actually use of IO.

2. Instead of locking the device during allocation of the loopback
   device, then unlocking it (which will make udev run), and then
   re-locking things if we need, let's instead just keep the lock the
   whole time, to make things a bit safer and faster, and not have to
   wait for udev at all. This is done by adding a "lock_op" parameter to
   loop device allocation functions that declares the initial state of
   the lock, and is one of LOCK_UN/LOCK_SH/LOCK_EX. This change also
   shortens a lot of code, since we allocate + immediately lock loopback
   devices pretty much everywhere.

1 files changed, 2 insertions, 11 deletions

diff --git a/src/dissect/dissect.c b/src/dissect/dissect.c
index 858ed6a8f9..f0094a390f 100644
--- a/src/dissect/dissect.c
+++ b/src/dissect/dissect.c
@@ -885,14 +885,10 @@ static int action_umount(const char *path) {
                 return log_error_errno(r, "Failed to get devname of block device " DEVNUM_FORMAT_STR ": %m",
                                        DEVNUM_FORMAT_VAL(devno));
 
-        r = loop_device_open(devname, 0, &d);
+        r = loop_device_open(devname, 0, LOCK_EX, &d);
         if (r < 0)
                 return log_error_errno(r, "Failed to open loop device '%s': %m", devname);
 
-        r = loop_device_flock(d, LOCK_EX);
-        if (r < 0)
-                return log_error_errno(r, "Failed to lock loop device '%s': %m", devname);
-
         /* We've locked the loop device, now we're ready to unmount. To allow the unmount to succeed, we have
          * to close the O_PATH fd we opened earlier. */
         fd = safe_close(fd);
@@ -944,16 +940,11 @@ static int run(int argc, char *argv[]) {
                         arg_image,
                         FLAGS_SET(arg_flags, DISSECT_IMAGE_DEVICE_READ_ONLY) ? O_RDONLY : O_RDWR,
                         FLAGS_SET(arg_flags, DISSECT_IMAGE_NO_PARTITION_TABLE) ? 0 : LO_FLAGS_PARTSCAN,
+                        LOCK_SH,
                         &d);
         if (r < 0)
                 return log_error_errno(r, "Failed to set up loopback device for %s: %m", arg_image);
 
-        /* Make sure udevd doesn't issue BLKRRPART underneath us thus making devices disappear in the middle,
-         * that we assume already are there. */
-        r = loop_device_flock(d, LOCK_SH);
-        if (r < 0)
-                return log_error_errno(r, "Failed to lock loopback device: %m");
-
         r = dissect_image_and_warn(
                         d->fd,
                         arg_image,