Re: [PATCH v2] locks: eliminate false positive conflicts for write lease

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

On Thu, Jun 13, 2019 at 04:13:15PM +0200, Miklos Szeredi wrote:
> On Wed, Jun 12, 2019 at 8:31 PM J . Bruce Fields <bfields@fieldses.org> wrote:
> >
> > How do opens for execute work?  I guess they create a struct file with
> > FMODE_EXEC and FMODE_RDONLY set and they decrement i_writecount.  Do
> > they also increment i_readcount?  Reading do_open_execat and alloc_file,
> > looks like it does, so, good, they should conflict with write leases,
> > which sounds right.
> 
> Right, but then why this:
> 
> > > +     /* Eliminate deny writes from actual writers count */
> > > +     if (wcount < 0)
> > > +             wcount = 0;
> 
> It's basically a no-op, as you say.  And it doesn't make any sense
> logically, since denying writes *should* deny write leases as well...

Yes.  I feel like the negative writecount case is a little nonobvious,
so maybe replace that by a comment, something like this?:

--b.

diff --git a/fs/locks.c b/fs/locks.c
index 2056595751e8..379829b913c1 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -1772,11 +1772,12 @@ check_conflicting_open(struct file *filp, const long arg, int flags)
 	if (arg == F_RDLCK && wcount > 0)
 		return -EAGAIN;
 
-	/* Eliminate deny writes from actual writers count */
-	if (wcount < 0)
-		wcount = 0;
-
-	/* Make sure that only read/write count is from lease requestor */
+	/*
+	 * Make sure that only read/write count is from lease requestor.
+	 * Note that this will result in denying write leases when wcount
+	 * is negative, which is what we want.  (We shouldn't grant
+	 * write leases on files open for execution.)
+	 */
 	if (filp->f_mode & FMODE_WRITE)
 		self_wcount = 1;
 	else if (filp->f_mode & FMODE_READ)