linux-kernel - Re: [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock

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: <20150720134849.GC3717@linux.vnet.ibm.com>
Date:	Mon, 20 Jul 2015 06:48:49 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Will Deacon <will.deacon@....com>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH v2] memory-barriers: remove
 smp_mb__after_unlock_lock()

On Mon, Jul 20, 2015 at 02:39:06PM +0100, Will Deacon wrote:
> On Fri, Jul 17, 2015 at 11:14:14PM +0100, Benjamin Herrenschmidt wrote:
> > On Fri, 2015-07-17 at 10:32 +0100, Will Deacon wrote:
> > >  static inline void arch_spin_unlock(arch_spinlock_t *lock)
> > >  {
> > > -       SYNC_IO;
> > > -       __asm__ __volatile__("# arch_spin_unlock\n\t"
> > > -                               PPC_RELEASE_BARRIER: : :"memory");
> > > +       smp_mb();
> > >         lock->slock = 0;
> > >  }
> > 
> > That probably needs to be mb() in case somebody has the expectation that
> > it does a barrier vs. DMA on UP.
> 
> Hmm, but on !SMP doesn't arch_spin_unlock effectively expand to barrier()
> in the core code?

Yes, to barrier(), but that doesn't generate any code.  In contrast, the
mb() that Ben is asking for puts out a sync instruction.  Without that
sync instruction, MMIO accesses can be reordered with the spin_unlock(),
even on single-CPU systems.  So the bm() is really needed if unlock is
to order against MMIO (and thus DMA) on UP.

							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/