linux-kernel - Re: memory barrier question

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 for Android: free password hash cracker in your pocket

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

Message-ID: <1284864421.30449.113.camel@pasglop>
Date:	Sun, 19 Sep 2010 12:47:01 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Miklos Szeredi <miklos@...redi.hu>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	dhowells@...hat.com, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org
Subject: Re: memory barrier question

On Fri, 2010-09-17 at 16:12 -0700, Paul E. McKenney wrote:
> On Sat, Sep 18, 2010 at 07:49:08AM +1000, Benjamin Herrenschmidt wrote:
> > 
> > > Right but in the concrete namei example I can't see how a compiler
> > > optimization can make a difference.  The order of the loads is quite
> > > clear:
> > > 
> > >    LOAD inode = next.dentry->inode
> > >    if (inode != NULL)
> > >    	LOAD inode->f_op
> > > 
> > > What is there the compiler can optimize?
> > 
> > Those two loads depend on each other, I don't think any implementation
> > can re-order them. In fact, such data dependency is typically what is
> > used to avoid having barriers in some cases. The second load cannot be
> > issued until the value from the first one is returned.
> 
> Sufficiently sadistic compiler and CPU implementations could do value
> speculation, for example, driven by profile-feedback optimization.
> Then the guess might initially incorrect, but then a store by some other
> CPU could make the subsequent test decide (wrongly) that the guess had
> in fact been correct.
> 
> Needless to say, I am not a fan of value speculation.  But other people
> do like it a lot.

Well, this verges on insanity... we get to a point where nobody's going
to get any code right :-)

I don't think the powerpc arch allows that, that leaves us with the
compiler, but so far I don't think gcc is -that- crazy. Those constructs
are common enough...

Cheers,
Ben.


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/