linux-kernel - Re: atomics: document that linux expects certain atomic behaviour from unsigned long

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: <200901060325.13362.nickpiggin@yahoo.com.au>
Date:	Tue, 6 Jan 2009 03:25:12 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, Pavel Machek <pavel@...e.cz>,
	kernel list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>, mtk.manpages@...il.com,
	rdunlap@...otime.net, linux-doc@...r.kernel.org,
	segher@...nel.crashing.org, rth@....gnu.org
Subject: Re: atomics: document that linux expects certain atomic behaviour from unsigned long

On Tuesday 06 January 2009 03:05:01 Paul E. McKenney wrote:
> On Mon, Jan 05, 2009 at 11:00:24PM +1100, Nick Piggin wrote:
> > On Monday 05 January 2009 22:23:50 Alan Cox wrote:
> > > > Pretty much everywhere that uses RCU for example does so using atomic
> > > > pointer loads and stores. The nastiest issue IMO actually is
> > > > reloading the value through the pointer even if it isn't explicitly
> > > > dereferenced. RCU gets this right with ACCESS_ONCE. Probably a lot of
> > > > code using basic types does not. x86 atomic_read maybe should be
> > > > using ACCESS_ONCE too...
> > >
> > > I'm pretty sure it should. gcc makes no guarantees about not being
> > > clever with accesses.
> >
> > Arguably it should. I don't know what the concurrent C standard looks
> > like, but prohibiting reloads of potentially concurrently modified memory
> > when there is no explicit pointer dereference is the natural complement
> > to prohibiting stores to potentially concurrently read memory when there
> > is no explicit store (which I think is begrudgingly agreed to be a
> > problem).
> >
> > http://lkml.org/lkml/2007/10/24/673
> >
> > I think I would like to see multiple reloads to local variables
> > prohibited, to avoid potential really subtle problems... But if
> > ACCESS_ONCE is here to stay, then I do think that atomic_read etc should
> > use it.
>
> The concurrency stuff in c++0x still permits the compiler to have its
> way with loads and stores to normal variables, but provides an "atomic"
> type that must be loaded and stored as specified in the program.

So:
if (trylock())
  locked = 1;
...
if (locked)
  *var = blah;
...
if (locked)
  unlock();

So the second part can still be transformed into a predicated calculation
of blah, then an unconditional store to *var?


> The issue with ACCESS_ONCE() is that gcc doesn't do any optimizations on
> volatile accesses, even the obvious ones.  Speaking of which, the gcc
> guys kicked out my bug 33102, which was complaining about this
> situation.  :-/

Hmm. It's still quite annoying even to have to switch everything to the
atomic type. I guarantee there will be bugs in Linux caused by the
compiler reloading pointers/longs/ints to access local variables...


--
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/