[adding back samba/nfs and fsdevel] On Fri, Apr 26, 2019 at 6:22 PM Jeff Layton <jlayton@xxxxxxxxxx> wrote: > > On Fri, 2019-04-26 at 10:50 -0400, J. Bruce Fields wrote: > > On Fri, Apr 26, 2019 at 04:11:00PM +0200, Amir Goldstein wrote: > > > On Fri, Apr 26, 2019, 4:00 PM J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote: > > > > > > > On Fri, Apr 26, 2019 at 03:50:46PM +0200, Amir Goldstein wrote: > > > > > On Fri, Feb 8, 2019, 5:03 PM Jeff Layton <jlayton@xxxxxxxxxx> wrote: > > > > > > Share/deny open semantics are pretty similar across NFS and SMB (by > > > > > > design, really). If you intend to solve that use-case, what you really > > > > > > want is whole-file, shared/exclusive locks that are set atomically with > > > > > > the open call. O_EXLOCK and O_SHLOCK seem like a reasonable fit there. > > > > > > > > > > > > Then you could have SMB and NFS servers set these flags when opening > > > > > > files, and deal with the occasional denial at open time. Other > > > > > > applications won't be aware of them of course, but that's probably fine > > > > > > for most use-cases where you want this sort of protocol interop. > > > > > > > > > > Sorry for posting off list. Airport emails... > > > > > I looked at implemeting O_EXLOCK and O_SHLOCK and it looks doable. > > > > > > > > > > I was wondering if there is an inherent reason not to allow an exclusive > > > > > lock on a file that is open read-only. > > > > > > > > > > Samba seems to need it and currently flock and ofd locks won't allow it. > > > > > Do you thing it will be ok to allow it with O_EXLOCK? > > > > > > > > Somebody could deny everyone access to a shared resource that everyone > > > > needs to make progress, like /etc/passwd or a shared library. > > > > > > > > Have you looked at Pavel Shilovsky's O_DENY patches? He had the feature > > > > off by default, with a mount option provided to turn it on. > > > > > > > > > > O_EXLOCK is advisory. It only aquired flock or ofd lock atomically with > > > open. > > > > Whoops, got it. > > > > Is that really adequate for open share locks, though? > > > > I assumed that Windows apps depend on the assumption that they're > > mandatory. So e.g. if you can get a DENY_READ open on a shared library > > then you know you can update it without the risk of making someone else > > crash. > > > > I think this is (slightly) better than doing it internally like we do > today and would give you coherent locking between NFS and SMB. Other > applications wouldn't see them, but for a NAS-style deployment, that's > probably ok. > We can do a little bit better. We can make sure that O_DENY_WRITE (named for convenience) fails if file is currently open for write by anyone and similarly for O_DENY_READ. But if we cannot deny future non-cooperative opens what's the point?.... > Any open by samba or nfsd would need to start setting O_SHLOCK, and deny > mode opens would have to set O_EXLOCK. We would actually need 2 per > inode though (one for read and one for write). > ...the point is that O_DENY_NONE does not need to be implemented with a new type of lock object (O_WR_SHLOCK) its enough that it checks there are no relevant exclusive locks and the then inode->i_writecount and inode->i_readcount already provide enough context to cooperate with O_DENY_WRITE and O_DENY_READ. I need to see if incrementing inode->i_readcount on O_RDWR opens is possible (right now it only counts O_RDONLY opens). > I think these should probably be in their own "namespace" too. They > could use the same semantics as flock, but should sit on their own list > in file_lock_context. > I would much rather that they didn't. The reason is that new open flags are a backward compat problem. The way I want to solve it is this API: // On new kernel this will acquire OFD F_WRLCK atomically... fd = open(..., O_RDWR | O_EXLOCK); // ...check if it did acquire OFD lock fcntl(fd, F_OFD_GETLK, ...); We'd need at least one new l_type F_EX_RDLCK and maybe also a new semantic F_EX_RDWRLCK, although similar in conflicts to F_WRLCK it can be acquired without FMODE_WRITE. Though I personally thing we can do without it if the only way to acquire F_WRLCK on readonly file is via new open flag. > That said, we could also look at a vfs-level mount option that would > make the kernel enforce these for any opener. That could also be useful, > and shouldn't be too hard to implement. Maybe even make it a vfsmount- > level option (like -o ro is). > Yeh, I am humbly going to leave this struggle to someone else. Not important enough IMO and completely independent effort to the advisory atomic open&lock API. > If you're denied, what error should you get back when you try to open > it? It should be something distinct. We may even want to add new error > codes for this. IMO EBUSY does the job. Its distinct because open is not expected to return EBUSY for regular files/dirs and when open is expected to return EBUSY for blockdev its for the exact same use case (i.e. exclusive write open is acquired by userspace tools). Thanks, Amir.
