[PATCH v2 3/8] vfs: Add O_DENYREAD/WRITE flags support for open syscall

[J. Bruce Fields]

On Tue, Feb 05, 2013 at 03:45:31PM +0400, Pavel Shilovsky wrote:
> 2013/1/31 J. Bruce Fields <bfields at fieldses.org>:
> > On Thu, Jan 17, 2013 at 08:52:59PM +0400, Pavel Shilovsky wrote:
> >> If O_DENYMAND flag is specified, O_DENYREAD/WRITE/MAND flags are
> >> translated to flock's flags:
> >>
> >> !O_DENYREAD  -> LOCK_READ
> >> !O_DENYWRITE -> LOCK_WRITE
> >> O_DENYMAND   -> LOCK_MAND
> >>
> >> and set through flock_lock_file on a file.
> >>
> >> This change only affects opens that use O_DENYMAND flag - all other
> >> native Linux opens don't care about these flags. It allow us to
> >> enable this feature for applications that need it (e.g. NFS and
> >> Samba servers that export the same directory for Windows clients,
> >> or Wine applications that access the same files simultaneously).
> >
> > The use of an is_conflict callback seems unnecessarily convoluted.
> >
> > If we need two different behaviors, let's just use another flag (or an
> > extra boolean argument if we need to, or something).
> 
> Ok, we can pass "bool is_mand" to flock_lock_file that will pass it
> further to flock_locks_conflict.
> 
> >
> > The only caller for this new deny_lock_file is in the nfs code--I'm a
> > little unclear why that is.
> 
> deny_lock_file is called not only in the nfs code but also in 2 places
> of fs/namei.c -- that enable this logic for VFS.

Oops, apologies, I overlooked those somehow.

What prevents somebody else from grabbing a lock on a newly-created file
before we grab our own lock?

I couldn't tell on a quick look whether we hold some lock that prevents
that.

--b.