Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Sun, Sep 13, 2015 at 12:06:33PM +0200, Patrick Marlier wrote:
> 
> On 09/12/2015 01:05 AM, Paul E. McKenney wrote:
> >On Tue, May 19, 2015 at 03:07:25PM -0700, Paul E. McKenney wrote:
> >>On Mon, May 18, 2015 at 09:43:21AM -0400, Steven Rostedt wrote:
> >>>On Mon, 18 May 2015 12:06:47 +1000
> >>>NeilBrown <neilb@suse.de> wrote:
> >>>
> >>>
> >>>>>struct mddev {
> >>>>>...
> >>>>>	struct list_head                disks;
> >>>>>...}
> >>>>>
> >>>>>struct list_head {
> >>>>>          struct list_head *next, *prev;
> >>>>>};
> >>>>>
> >>>>>The tricky thing is that "list_entry_rcu" before and after the patch is
> >>>>>reading the same thing.
> >>>>
> >>>>No it isn't.
> >>>>Before the patch it is passed the address of the 'next' field.  After the
> >>>>patch it is passed the contents of the 'next' field.
> >>>
> >>>Right.
> >>>
> >>>>
> >>>>
> >>>>>
> >>>>>However in your case, the change I proposed is probably wrong I trust
> >>>>>you on this side. :) What's your proposal to fix it with the rculist patch?
> >>>>
> >>>>What needs fixing?  I don't see anything broken.
> >>>>
> >>>>Maybe there is something in this "rculist patch" that I'm missing.  Can you
> >>>>point me at it?
> >>>>
> >>>
> >>>Probably some debugging tool like sparse notices that the assignment
> >>>isn't a true list entry and complains about it. In other words, I think
> >>>the real fix is to fix the debugging tool to ignore this, because the
> >>>code is correct, and this is a false positive failure, and is causing
> >>>more harm than good, because people are sending out broken patches due
> >>>to it.
> >>
> >>OK, finally did the history trawling that I should have done to begin with.
> >>
> >>Back in 2010, Arnd added the __rcu pointer checking in sparse.
> >>But the RCU list primitives were used on non-RCU-protected lists, so
> >>some casting pain was required to avoid sparse complaints.  (Keep in
> >>mind that the list_head structure does not mark ->next with __rcu.)
> >>Arnd's workaround was to copy the pointer to the local stack, casting
> >>it to an __rcu pointer, then use rcu_dereference_raw() to do the needed
> >>traversal of an RCU-protected pointer.
> >>
> >>This of course resulted in an extraneous load from the stack, which
> >>Patrick noticed in his performance work, and which motivated him to send
> >>the patches.
> >>
> >>Perhaps what I should do is create an rcu_dereference_nocheck() for use
> >>in list traversals, that omits the sparse checking.  That should get rid
> >>of both the sparse warnings and the strange casts.
> >>
> >>The code in md probably needs to change in any case, as otherwise we are
> >>invoking rcu_dereference_whatever() on a full struct list_head rather
> >>than on a single pointer.  Or am I missing something here?
> >
> >Finally getting back to this one...
> >
> >I switched to lockless_dereference() instead of rcu_dereference_raw(),
> >and am running it through the testing gamut.  Patrick, are you OK with
> >this change?
> 
> Paul,
> 
> This sounds good to me. It should fix the performance issue (will
> check with my benchmark).

Thank you, looking forward to seeing the results!

> I think for drivers/md/bitmap.c:next_active_rdev() the problem was
> fixed but do you know if it also fixed for
> net/netfilter/core.c:nf_hook_slow()?

It does appear so.  The statement now reads:

	elem = list_entry_rcu(state->hook_list, struct nf_hook_ops, list);

And ->hook_list is defined as follows:

	struct list_head *hook_list;

							Thanx, Paul