I've had a look at nbd driver code and viewed the trace log, and get clear
about why the previously mentioned problem happens:
1st time: qemu-nbd -c /dev/nbd0 disk.img
nbd_init: send these ioctl(s) in order, SET_BLKSIZE, SET_SIZE, CLEAR_SOCK,
SET_SOCK
nbd_clinet: NBD_DO_IT (it will then handle request(s) in which it should use
nbd_device->sock.)
2st time: qemu-nbd -c /dev/nbd0 disk1.img
nbd_init: send same ioctl(s) to the same nbd device, it will reset
nbd_device->sock
nbd_client: still send NBD_DO_IT, it find there is on client connected, then
return EBUSY and send CLEAR_SOCK, the result is: it will clear
ndb_device->sock, which will cause the 1st time "qemu-nbd -c" fail to handle
request any longer, including unable to read partition table.
According to above code logic, if lock in an early place is not accepted, then
removing CLEAR_SOCK in nbd_init phase can also solve problem. In fact, if
cleanup work done well, I think that ioctl is not needed. Any comments?
diff --git a/nbd.c b/nbd.c
index e6c931c..067a57b 100644
--- a/nbd.c
+++ b/nbd.c
@@ -386,15 +386,6 @@ int nbd_init(int fd, int csock, uint32_t flags, off_t
size, size_t blocksize)
return -1;
}
- TRACE("Clearing NBD socket");
-
- if (ioctl(fd, NBD_CLEAR_SOCK) == -1) {
- int serrno = errno;
- LOG("Failed clearing NBD socket");
- errno = serrno;
- return -1;
- }
-
TRACE("Setting NBD socket");
if (ioctl(fd, NBD_SET_SOCK, csock) == -1) {
>>> Paolo Bonzini <[email protected]> 11/18/2011 4:59 PM >>>
On 11/18/2011 02:25 AM, Chun Yan Liu wrote:
> Yes. I have tested using same device twice as described in my previous
> mail. Without lock:
>
> If issuing "qemu-nbd -c /dev/nbd0 disk.img" and "qemu-nbd -c /dev/nbd0
> disk1.img" almost at the same time, both can pass nbd_init() and get to
> nbd_client(), then the latter one will fail and exit, but the first one
> does not work well either (fail to show partitions.) That's why I think
> we should add a lock in an earlier time.
This is an initialization problem. As Stefan wrote, functionality for
atomic acquisition of NBD devices is provided by the kernel; the problem
is simply that QEMU does not use it. :)
Paolo
