Re: Better interop for NFS/SMB file share mode/reservation

["J. Bruce Fields" <bfields@fieldses.org>]

On Fri, Feb 15, 2019 at 09:31:48AM +0200, Amir Goldstein wrote:
> On Thu, Feb 14, 2019 at 10:51 PM J. Bruce Fields <bfields@fieldses.org> wrote:
> >
> > On Fri, Feb 08, 2019 at 10:31:07PM +0200, Amir Goldstein wrote:
> > > On Fri, Feb 8, 2019 at 10:17 PM J. Bruce Fields <bfields@fieldses.org> wrote:
> > > > On Fri, Feb 08, 2019 at 10:02:43PM +0200, Amir Goldstein wrote:
> > > > > On Fri, Feb 8, 2019 at 5:51 PM J. Bruce Fields <bfields@fieldses.org> wrote:
> > > > > > On Fri, Feb 08, 2019 at 04:45:46PM +0200, Amir Goldstein wrote:
> > > > > > > - check_conflicting_open() is changed to use inode_is_open_for_read()
> > > > > > >   instead of checking d_count and i_count.
> > > > > >
> > > > > > Independently of the rest, I'd love to do away with those
> > > > > > d_count/i_count checks.  What's inode_is_open_for_read()?
> > > > > >
> > > > >
> > > > > It would look maybe something like this:
> > > > >
> > > > > static inline bool file_is_open_for_read(const struct inode *file)
> > > > > {
> > > > >         struct inode *inode = file_inode(file);
> > > > >         int countself = (file->f_mode & (FMODE_READ | FMODE_WRITE)) ==
> > > > > FMODE_READ) ? 1 : 0;
> > > > >
> > > > >         return atomic_read(&inode->i_readcount) > countself;
> > > > > }
> > > > >
> > > > > And it would allow for acquiring F_WRLCK lease if other
> > > > > instances of inode are open O_PATH.
> > > > > A slight change of semantics that seems harmless(?)
> > > > > and will allow some flexibility.
> > > >
> > > > How did I not know about i_readcount?  (Looking)  I guess it would mean
> > > > adding some dependence on CONFIG_IMA, hm.
> > > >
> > >
> > > Yes, or we remove ifdef CONFIG_IMA from i_readcount.
> > > I am not sure if the concern was size of struct inode
> > > (shouldn't increase on 64bit arch) or the accounting on
> > > open/close. The impact doesn't look significant (?)..
> >
> > Looks like the original patch was d984ea604943bb "fs: move i_readcount".
> > I did some googling around and looked at the discussion summarized by
> > https://lwn.net/Articles/410895/ but can't find useful discussion of
> > i_readcount impact.
> >
> > Looks like CONFIG_IMA is on in Fedora and RHEL, for what it's worth.
> >
> > Maybe something like this?
> >
> > --b.
> >
> > commit 02cfda99ed8c
> > Author: J. Bruce Fields <bfields@redhat.com>
> > Date:   Thu Feb 14 15:02:02 2019 -0500
> >
> >     locks: use i_readcount to detect lease conflicts
> >
> >     The lease code currently uses the inode and dentry refcounts to detect
> >     whether someone has a file open for read.  This seems fragile.  Use
> >     i_readcount instead.
> >
> >     Signed-off-by: J. Bruce Fields <bfields@redhat.com>
> >
> > diff --git a/fs/locks.c b/fs/locks.c
> > index ff6af2c32601..299abad65545 100644
> > --- a/fs/locks.c
> > +++ b/fs/locks.c
> > @@ -1769,8 +1769,7 @@ check_conflicting_open(const struct dentry *dentry, const long arg, int flags)
> >         if ((arg == F_RDLCK) && inode_is_open_for_write(inode))
> >                 return -EAGAIN;
> >
> > -       if ((arg == F_WRLCK) && ((d_count(dentry) > 1) ||
> > -           (atomic_read(&inode->i_count) > 1)))
> > +       if ((arg == F_WRLCK) && (atomic_read(&inode->i_readcount) > 1))
> >                 ret = -EAGAIN;
> 
> Alas, i_readcount is not the count of file opens for read, it is the count
> of file opens O_RDONLY, so this is incorrect wrt conflict with other writers.

Whoops, thanks!

> I guess since there is a full smp_mb() before this check, then you
> can check (i_readcount + i_writecount) > 1 || (i_writecount < 0)
> 
> You can also check if caller itself is O_RDONLY to know if self
> count is expect to be in i_readcount or i_writecount, but not sure
> it is worth the trouble.

I don't know, it still looks reasonable.  I'll fool around with it.

--b.