linux-kernel - Re: [PATCH tip/core/rcu 09/12] fs: Substitute rcu_access_pointer() for rcu_dereference

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Message-ID: <20140218000431.GB19929@thin>
Date:	Mon, 17 Feb 2014 16:04:31 -0800
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	darren@...art.com, fweisbec@...il.com, oleg@...hat.com,
	sbw@....edu, Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH tip/core/rcu 09/12] fs: Substitute rcu_access_pointer()
 for rcu_dereference_raw()

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@...ux.vnet.ibm.com>
> > > 
> > > (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@...ux.vnet.ibm.com>
> > > Cc: Alexander Viro <viro@...iv.linux.org.uk>
> > > Cc: linux-fsdevel@...r.kernel.org
> > 
> > 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@...htriplett.org>
> > 
> > >  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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