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

J. Bruce Fields
2019-02-14T20:51:02Z

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;

return ret;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 29d8e2cfed0e..e862de682da9 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -676,7 +676,7 @@ struct inode {
atomic_t i_count;
atomic_t i_dio_count;
atomic_t i_writecount;
-#ifdef CONFIG_IMA
+#if defined(CONFIG_IMA) || (defined_CONFIG_FILE_LOCKING)
atomic_t i_readcount; /* struct files open RO */
#endif
const struct file_operations *i_fop; /* former ->i_op->default_file_ops */
@@ -2869,7 +2869,7 @@ static inline bool inode_is_open_for_write(const struct inode *inode)
return atomic_read(&inode->i_writecount) > 0;
}

-#ifdef CONFIG_IMA
+#if defined(CONFIG_IMA) || defined(CONFIG_FILE_LOCKING)
static inline void i_readcount_dec(struct inode *inode)
{
BUG_ON(!atomic_read(&inode->i_readcount));