linux-kernel - Re: sequence lock in Linux

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: <20100611200700.GG2394@linux.vnet.ibm.com>
Date:	Fri, 11 Jun 2010 13:07:00 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: sequence lock in Linux

On Fri, Jun 11, 2010 at 03:40:16PM -0400, Mathieu Desnoyers wrote:
> Hi Paul,
> 
> (CCing lkml)
> 
> Is it just me, or the following code:
> 
> static __always_inline unsigned read_seqbegin(const seqlock_t *sl)
> {
>         unsigned ret;
> 
> repeat:
>         ret = sl->sequence;
>         smp_rmb();
>         if (unlikely(ret & 1)) {
>                 cpu_relax();
>                 goto repeat;
>         }
> 
>         return ret;
> }
> 
> could use a ACCESS_ONCE() around the sl->sequence read ? I'm concerned about the
> compiler generating code that reads the sequence number chunkwise.
> 
> The same apply to all other reads of the sequence number in seqlock.h (including
> the retry code).
> 
> Thoughts ?

Doesn't gcc guarantee that accesses to aligned basic types that fit into
a machine word are loaded and stored in one shot?  Now, gcc might choose
to load twice (or to merge loads) due to things like register pressure,
but given that ->sequence is an int, gcc should not be accessing it
(say) bytewise on any platform supporting 32-bit accesses.

Or am I suffering from wishful thinking here?

							Thanx, Paul
--
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/