On Mon, Mar 22, 2021 at 09:27:38PM +0300, Vladimir Sementsov-Ogievskiy wrote: > Let's document how we use file locks in file-posix driver, to allow > external programs to "communicate" in this way with Qemu. > > Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]> > --- > > Hi all! > > We need to access disk images from non-Qemu code and coordinate with > Qemu utilities which may use same image. So, we want to support Qemu > file locking in the external code. > > So, here is a patch to document how Qemu locking works, and make this > thing "public". > > This is an RFC, because I'm unsure how should we actually document > different operations we have. > > For example greaph-mod is a strange thing, I think we should get rid > of it at all.. And at least, no sense in locking corresponding byte in a > raw file. > > The other thing is write-unchanged.. What it means when we consider a > raw file opened in several processes? Probably we don't need it too.. > > docs/system/qemu-block-drivers.rst.inc | 55 ++++++++++++++++++++++++++ > 1 file changed, 55 insertions(+) > > diff --git a/docs/system/qemu-block-drivers.rst.inc > b/docs/system/qemu-block-drivers.rst.inc > index b052a6d14e..3cd708b3dc 100644 > --- a/docs/system/qemu-block-drivers.rst.inc > +++ b/docs/system/qemu-block-drivers.rst.inc > @@ -952,3 +952,58 @@ on host and see if there are locks held by the QEMU > process on the image file. > More than one byte could be locked by the QEMU instance, each byte of which > reflects a particular permission that is acquired or protected by the running > block driver. > + > +Image locking protocol > +~~~~~~~~~~~~~~~~~~~~~~ > + > +QEMU holds rd locks and never rw locks. Instead, GETLK fcntl is used with > F_WRLCK > +to handle permissions as described below. > +QEMU process may rd-lock the following bytes of the image with corresponding > +meaning: > + > +Permission bytes. If permission byte is rd-locked, it means that some process > +uses corresponding permission on that file. > + > +Byte Operation > +100 read > + Lock holder can read > +101 write > + Lock holder can write > +102 write-unchanged > + Lock holder can write same data > +103 resize > + Lock holder can resize the file > +104 graph-mod > + Undefined. QEMU sometimes locks this byte, but external programs > + should not. QEMU will stop locking this byte in future > + > +Unshare bytes. If permission byte is rd-locked, it means that some process > +does not allow the others use corresponding options on that file. > + > +Byte Operation > +200 read > + Lock holder don't allow read operation to other processes. > +201 write > + Lock holder don't allow write operation to other processes. > +202 write-unchanged > + Lock holder don't allow write-unchanged operation to other > processes. > +203 resize > + Lock holder don't allow resizing the file by other processes. > +204 graph-mod > + Undefined. QEMU sometimes locks this byte, but external programs > + should not. QEMU will stop locking this byte in future > + > +Handling the permissions works as follows: assume we want to open the file > to do > +some operations and in the same time want to disallow some operation to other > +processes. So, we want to lock some of the bytes described above. We operate > as > +follows: > + > +1. rd-lock all needed bytes, both "permission" bytes and "unshare" bytes. > + > +2. For each "unshare" byte we rd-locked, do GETLK that "tries" to wr-lock > +corresponding "permission" byte. So, we check is there any other process that > +uses the permission we want to unshare. If it exists we fail. > + > +3. For each "permission" byte we rd-locked, do GETLK that "tries" to wr-lock > +corresponding "unshare" byte. So, we check is there any other process that > +unshares the permission we want to have. If it exists we fail.
I'd perhaps illustrate the few common scenarios - read-only, shared access, read-write, exclusive and read-write share access, using C-psuedo-code, from opening image, locking, doing operations, unlocking and closing image. This would make it explicit that these locks need to be held open for the duration of the i/o operations. I'd also probably warn about the dangers of traditional fcntl locks in threaded programs, compared to the saner OFD locks. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
