Better interop for NFS/SMB file share mode/reservation

[Amir Goldstein]

On Thu, Feb 14, 2019 at 10:51 PM J. Bruce Fields <bfields at 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 at 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 at 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 at 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 at 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.

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.

Thanks,
Amir.