linux-kernel - Re: [PATCH rcu 6/6] rcu: Use WRITE_ONCE() for assignments to ->next for rculist

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Date:   Wed, 19 Jul 2023 13:02:58 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Alan Huang <mmpgouride@...il.com>
Cc:     Joel Fernandes <joel@...lfernandes.org>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...a.com,
        rostedt@...dmis.org
Subject: Re: [PATCH rcu 6/6] rcu: Use WRITE_ONCE() for assignments to ->next
 for rculist_nulls

On Thu, Jul 20, 2023 at 03:17:58AM +0800, Alan Huang wrote:
> 
> > 2023年7月20日 02:20，Paul E. McKenney <paulmck@...nel.org> 写道：
> > 
> > On Tue, Jul 18, 2023 at 09:48:59PM -0400, Joel Fernandes wrote:
> >> 
> >> 
> >> On 7/18/23 14:32, Paul E. McKenney wrote:
> >>> On Tue, Jul 18, 2023 at 10:48:07PM +0800, Alan Huang wrote:
> >>>> 
> >>>>> 2023年7月18日 21:49，Joel Fernandes <joel@...lfernandes.org> 写道：
> >>>>> 
> >>>>> On 7/17/23 14:03, Paul E. McKenney wrote:
> >>>>>> From: Alan Huang <mmpgouride@...il.com>
> >>>>>> When the objects managed by rculist_nulls are allocated with
> >>>>>> SLAB_TYPESAFE_BY_RCU, old readers may still hold references to an object
> >>>>>> even though it is just now being added, which means the modification of
> >>>>>> ->next is visible to readers.  This patch therefore uses WRITE_ONCE()
> >>>>>> for assignments to ->next.
> >>>>>> Signed-off-by: Alan Huang <mmpgouride@...il.com>
> >>>>>> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> >>>>> 
> >>>>> Did we ever conclude that the READ_ONCE() counterparts were not needed? ;-)
> >>>> 
> >>>> Read-side is already protected by rcu_dereference_raw() in hlist_nulls_for_each_entry_{rcu, safe}.
> >>> 
> >>> It turns out that different traversal synchronization designs want
> >>> different pointers using WRITE_ONCE().
> >> 
> >> Thank you Alan and Paul,
> >> 
> >> Btw, I don't see any users of hlist_nulls_unhashed_lockless(), maybe it can
> >> be removed?
> > 
> > Either that or the people who removed uses injected bugs...
> 
> It has never been used.
> 
> That said, the data race has been there almost for four years.
> 
> And the network people use sk_unhashed() for both hlist_node and hlist_nulls_node.
> So, I plan to use hlist_unhashed_lockless() in sk_unhashed(), that will be one of my future patches.
> 
> > 
> > But if this one really does go away, do we need ->pprev to be
> > protected by _ONCE()?
> 
> The ->pprev thing is what I’m currently working on. :)

Very good, looking forward to seeing what you come up with!

							Thanx, Paul