linux-kernel - Re: [RFC][PATCH v3]: documentation,atomic: Add new documents

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20170731110403.ou3zqsp3uviqorkz@tardis>
Date:   Mon, 31 Jul 2017 19:04:03 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Will Deacon <will.deacon@....com>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [RFC][PATCH v3]: documentation,atomic: Add new documents

On Mon, Jul 31, 2017 at 11:05:35AM +0200, Peter Zijlstra wrote:
> On Wed, Jul 26, 2017 at 08:47:50PM +0800, Boqun Feng wrote:
> 
> > > +
> > > +Further, while something like:
> > > +
> > > +  smp_mb__before_atomic();
> > > +  atomic_dec(&X);
> > > +
> > > +is a 'typical' RELEASE pattern, the barrier is strictly stronger than
> > > +a RELEASE. Similarly for something like:
> > > +
> > 
> > .. at here. Maybe you planned to put stronger ACQUIRE pattern?
> 
> Yes, although I struggled to find a sensible one. The problem is that
> ACQUIRE is on loads and value returning atomics have an ACQUIRE variant,
> so why would you ever want to use smp_mb__after_atomic() for this.
> 
> 
> That is, the best I could come up with is something like:
> 
> 	val = atomic_fetch_or_relaxed(1, &var);
> 	smp_mb__after_atomic();
> 
> But in that case we should've just written:
> 
> 	val = atomic_fetch_or_acquire(1, &var);
> 

Agreed.

And besides, in memory-barriers.txt, the wording is:

 (*) smp_mb__before_atomic();
 (*) smp_mb__after_atomic();

     These are for use with atomic (such as add, subtract, increment and
     decrement) functions that don't return a value, especially when used for
     reference counting. 

So actually, using smp_mb__after_atomic() for ACQUIRE is a misuse.

> 
> Suggestions?

As a result, I think it's better we say smp_mb__{before,after}_atomic()
are only for 1) non-value-returning RmW atomic ops, 2)
{set,clear,change}_bit and 3) internal use of atomic primitives(e.g. the
generic version of fully ordered atomics).

1) prevents people to use it for an ACQUIRE, but allows for a RELEASE.
1) & 2) makes atomic_t.txt consistent with memory-barriers.txt
3) explains our usage of those barriers internally.

Thoughts?

Regards,
Boqun

Download attachment "signature.asc" of type "application/pgp-signature" (489 bytes)