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

[Patrick Marlier]

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@xxxxxxx> 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).
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()?

Thanks.
--
Pat
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