Re: [Qemu-devel] Disable image locking for snapshot drive?

[Fam Zheng <famz@redhat.com>]

On Tue, 07/18 16:19, Andrew Baumann wrote:
> > From: Eric Blake [mailto:eblake@redhat.com]
> > Sent: Tuesday, 18 July 2017 8:07
> > On 07/17/2017 07:33 PM, John Snow wrote:
> > > On 07/17/2017 07:30 PM, Andrew Baumann via Qemu-devel wrote:
> > >> I'm running a recent Linux build of qemu on Windows Subsystem for Linux
> > (WSL) which doesn't appear to implement file locking:
> > >>
> > >> $ qemu-system-aarch64 ... -drive file=test.vhdx,if=none,id=hd0 -device
> > virtio-blk-pci,drive=hd0
> > >> qemu-system-aarch64: -drive file=test.vhdx,if=none,id=hd0: Failed to
> > unlock byte 100
> > 
> > Does WSL implement fcntl(F_SETLK) but not fcntl(F_OFD_SETLK)?
> 
> Yes, this appears to be the case (there's also one report that it's broken):
> https://github.com/Microsoft/BashOnWindows/issues/1927

What does fcntl(F_OFD_SETLK) return? If it is -ENOTSUP, we can probably detect
that and disable locking. Can you try the patch pasted in the end?

> 
> > We
> > already have code in place for compiling when F_OFD_SETLK is not
> > supported (which makes lock=auto do nothing, and issues a warning that
> > F_SETLK locks may be ineffective when locks are explicitly requested),
> > do we need to just expand that code into a runtime test of whether
> > F_OFD_SETLK appears to be unsupported?
> 
> That would be a nice fix, and it would avoid the need for yet another flag. On
> the other hand, WSL is aiming for ABI compatibility, so they should get around
> to implementing F_OFD_SETLK et al eventually.
> 
> Even if this were fixed in QEMU or implemented in WSL, shouldn't there to be a
> way to turn snapshot file locking off on a per-drive basis?

A snapshot file is temporary and unlinked immediately, so applying a lock or not
doesn't matter that much to deserve an option for that.

---

diff --git a/block/file-posix.c b/block/file-posix.c
index cfbb236f6f..0be5bbbd53 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -493,6 +493,12 @@ static int raw_open_common(BlockDriverState *bs, QDict *options,
     }
     s->fd = fd;
 
+    if (s->use_lock) {
+        int ret0 = qemu_unlock_fd(fd, 0, 0);
+        if (ret0 == -ENOTSUP) {
+            s->use_lock = false;
+        }
+    }
     s->lock_fd = -1;
     if (s->use_lock) {
         fd = qemu_open(filename, s->open_flags);