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

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Message-ID: <50bc6e0d-7d9b-4634-999a-2ee8f503e86a@paulmck-laptop>
Date:   Wed, 19 Jul 2023 11:20:36 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Alan Huang <mmpgouride@...il.com>, 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 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...

But if this one really does go away, do we need ->pprev to be
protected by _ONCE()?

							Thanx, Paul