On Tue, 07/18 16:19, Andrew Baumann wrote:
> > From: Eric Blake [mailto:ebl...@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);
