On Mon, Feb 17, 2014 at 03:05:11PM -0800, Paul E. McKenney wrote:
> On Mon, Feb 17, 2014 at 02:00:15PM -0800, Josh Triplett wrote:
> > On Mon, Feb 17, 2014 at 01:35:56PM -0800, Paul E. McKenney wrote:
> > > From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> > >
> > > (Trivial patch.)
> > >
> > > If the code is looking at the RCU-protected pointer itself, but not
> > > dereferencing it, the rcu_dereference() functions can be downgraded to
> > > rcu_access_pointer(). This commit makes this downgrade in __alloc_fd(),
> > > which simply compares the RCU-protected pointer against NULL with no
> > > dereferencing.
> > >
> > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > > Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> > > Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> >
> > I'm beginning to wonder if this common pattern ought to have an
> > rcu_pointer_is_null(), which would not return the pointer, only the
> > boolean.
>
> Or perhaps an rcu_compare_pointer() to also handle the various cases like:
>
> if (rcu_dereference_raw(foop) == barp) ...
>
> I added the problem to the RCU cleanup list on the OPW site, and
> your solution or my elaboration of it might be the right thing to do.
> (Inspected all 1300 uses of members of the rcu_dereference() family of
> functions last week, and was feeling a bit buggy-eyed at the end...)
rcu_pointer_eq and/or rcu_pointer_neq might make sense, yeah, as
self-documenting versions of the most sensible way to do the operation,
to steer people away from rcu_dereference or rcu_dereference_raw.
- Jsoh Triplett
> Thanx, Paul
>
> > Regardless, for this patch:
> > Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> >
> > > fs/file.c | 2 +-
> > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/fs/file.c b/fs/file.c
> > > index db25c2bdfe46..18f7d27855c4 100644
> > > --- a/fs/file.c
> > > +++ b/fs/file.c
> > > @@ -497,7 +497,7 @@ repeat:
> > > error = fd;
> > > #if 1
> > > /* Sanity check */
> > > - if (rcu_dereference_raw(fdt->fd[fd]) != NULL) {
> > > + if (rcu_access_pointer(fdt->fd[fd]) != NULL) {
> > > printk(KERN_WARNING "alloc_fd: slot %d not NULL!\n", fd);
> > > rcu_assign_pointer(fdt->fd[fd], NULL);
> > > }
> > > --
> > > 1.8.1.5
> > >
> >
>
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