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Message-ID: <f363d6e0-5682-43e7-9a3f-6b896c3cd920@paulmck-laptop>
Date: Thu, 19 Oct 2023 09:39:51 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
Cc: linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
linux-doc@...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>,
Joel Fernandes <joel@...lfernandes.org>,
Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH memory-model] docs: memory-barriers: Add note on compiler
transformation and address deps
On Wed, Oct 18, 2023 at 12:11:58PM +0200, Jonas Oberhauser wrote:
> Hi Paul,
>
> on a second thought. Why can't the compiler always do, e.g.,
>
> int *p = READ_ONCE(shared_ptr);
>
> assert (*p == 0);
>
> ~>
>
> int *p = READ_ONCE(shared_ptr);
>
> int val = x; // x is some object that definitely won't segfault, but may
> very well be owned by another thread right now
> if (p != &x) val = *p;
The compiler is forbidden from inventing pointer comparisons.
> assert (val == 0);
>
> and in case p == &x, the address dependency is elided
But yes, this is one reason why Documentation/RCU/rcu_dereference.rst
warns about pointer comparisons.
> Best wishes,
>
> jonas
>
> Am 10/6/2023 um 6:39 PM schrieb Jonas Oberhauser:
> > Hi Paul,
> >
> > The "more up-to-date information" makes it sound like (some of) the
> > information in this section is out-of-date/no longer valid.
The old smp_read_barrier_depends() that these section cover really
does no longer exist.
> > But after reading the sections, it seems the information is valid, but
> > discusses mostly the history of address dependency barriers.
> >
> > Given that the sepcond part specifically already starts with a
> > disclaimer that this information is purely relevant to people interested
> > in history or working on alpha, I think it would make more sense to
> > modify things slightly differently.
> >
> > Firstly I'd remove the "historical" part in the first section, and add
> > two short paragraphs explaining that
> >
> > - every marked access implies a address dependency barrier
This is covered in rcu_dereference.rst. Or is something missing there?
Please note that the atomic_read() primitives operate on integers
rather than pointers, so are off the table. Yes, in theory, some of
the value-returning atomic read-modify-write operations could head a
dependency chain, but these things are sufficiently heavyweight that
most situations would be better served by an _acquire() suffix than by
a relaxed version of such an atomic operation.
> > - address dependencies considered by the model are *semantic*
> > dependencies, meaning that a *syntactic* dependency is not sufficient to
> > imply ordering; see the rcu file for some examples where compilers can
> > elide syntactic dependencies
There is a bunch of text in rcu_dereference.rst to this effect. Or
is there some aspect that is missing from that document?
The longer-term direction, perhaps a few years from now, is for the
first section to simply reference rcu_dereference.rst and for the second
section to be removed completely.
> > Secondly, I'd not add the disclaimer to the second section; there's
> > already a link to rcu_dereference in that section ( https://github.com/torvalds/linux/blob/master/Documentation/memory-barriers.txt#L634
> > ), and already a small text explaining that the section is historical.
The problem is that people insist on diving into the middle of documents,
so sometimes repetition is a necessary form of self defense. ;-)
But I very much appreciate your review and feedback, and I also apologize
for my slowness.
Thanx, Paul
> > Best wishes,
> >
> > jonas
> >
> >
> > Am 10/5/2023 um 6:53 PM schrieb Paul E. McKenney:
> > > 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 in the beginning of both the
> > > historical address-dependency sections and point to rcu-dereference.rst
> > > for more information.
> > >
> > > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > >
> > > diff --git a/Documentation/memory-barriers.txt
> > > b/Documentation/memory-barriers.txt
> > > index 06e14efd8662..d414e145f912 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
> > > @@ -556,6 +560,9 @@ 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
>
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