linux-kernel - Re: [PATCH 2/2] docs: memory-barriers: Add note on plain-accesses to address-dependency barriers

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Date:   Fri, 4 Aug 2023 05:11:27 +0000
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH 2/2] docs: memory-barriers: Add note on plain-accesses to
 address-dependency barriers

On Thu, Aug 03, 2023 at 11:52:06AM -0700, Paul E. McKenney wrote:
> On Thu, Aug 03, 2023 at 03:24:07AM +0000, Joel Fernandes (Google) wrote:
> > The compiler has the ability to cause misordering by destroying
> > address-dependency barriers if comparison operations are used. Add a
> > note about this to memory-barriers.txt and point to rcu-dereference.rst
> > for more information.
> > 
> > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > ---
> >  Documentation/memory-barriers.txt | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> > index 06e14efd8662..acc8ec5ce563 100644
> > --- a/Documentation/memory-barriers.txt
> > +++ b/Documentation/memory-barriers.txt
> > @@ -435,6 +435,11 @@ Memory barriers come in four basic varieties:
> >       variables such as READ_ONCE() and rcu_dereference() provide implicit
> >       address-dependency barriers.
> >  
> > +     [!] Note that address dependency barriers can be destroyed by comparison
> > +     of a pointer obtained by a marked accessor such as READ_ONCE() or
> > +     rcu_dereference() with some value.  For an example of this, see
> > +     rcu_dereference.rst (part where the comparison of pointers is discussed).
> 
> Hmmm...
> 
> Given that this is in a section marked "historical" (for the old
> smp_read_barrier_depends() API), why not instead add a pointer to
> Documentation/RCU/rcu_dereference.rst to the beginning of the section,
> noted as the updated material?

Sounds good. There's also another section in the same file on Address
dependency barriers (also marked historical). So something like the
following?

---8<-----------------------

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index acc8ec5ce563..ba50220716ca 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -396,6 +396,10 @@ Memory barriers come in four basic varieties:
 
 
  (2) Address-dependency barriers (historical).
+     [!] This section is marked as HISTORICAL: For more up-to-date
+     information, including how compiler transformations related to pointer
+     comparisons can sometimes cause problems, see
+     Documentation/RCU/rcu_dereference.rst.
 
      An address-dependency barrier is a weaker form of read barrier.  In the
      case where two loads are performed such that the second depends on the
@@ -561,6 +565,9 @@ There are certain things that the Linux kernel memory barriers do not guarantee:
 
 ADDRESS-DEPENDENCY BARRIERS (HISTORICAL)
 ----------------------------------------
+[!] This section is marked as HISTORICAL: For more up-to-date information,
+including how compiler transformations related to pointer comparisons can
+sometimes cause problems, see Documentation/RCU/rcu_dereference.rst.
 
 As of v4.15 of the Linux kernel, an smp_mb() was added to READ_ONCE() for
 DEC Alpha, which means that about the only people who need to pay attention