linux-kernel - Re: [PATCH 4/4] locking: Introduce smp_cond

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: <20151117210109.GY5184@linux.vnet.ibm.com>
Date:	Tue, 17 Nov 2015 13:01:09 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Will Deacon <will.deacon@....com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jonathan Corbet <corbet@....net>,
	Michal Hocko <mhocko@...nel.org>,
	David Howells <dhowells@...hat.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH 4/4] locking: Introduce smp_cond_acquire()

On Tue, Nov 17, 2015 at 11:51:10AM +0000, Will Deacon wrote:
> Hi Linus,
> 
> On Mon, Nov 16, 2015 at 01:58:49PM -0800, Linus Torvalds wrote:
> > On Mon, Nov 16, 2015 at 8:24 AM, Will Deacon <will.deacon@....com> wrote:
> > >
> > > ... or we upgrade spin_unlock_wait to a LOCK operation, which might be
> > > slightly cheaper than spin_lock()+spin_unlock().
> > 
> > So traditionally the real concern has been the cacheline ping-pong
> > part of spin_unlock_wait(). I think adding a memory barrier (that
> > doesn't force any exclusive states, just ordering) to it is fine, but
> > I don't think we want to necessarily have it have to get the cacheline
> > into exclusive state.
> 
> The problem is, I don't think the memory-barrier buys you anything in
> the context of Boqun's example. In fact, he already had smp_mb() either
> side of the spin_unlock_wait() and its still broken on arm64 and ppc.
> 
> Paul is proposing adding a memory barrier after spin_lock() in the racing
> thread, but I personally think people will forget to add that.

A mechanical check would certainly make me feel better about it, so that
any lock that was passed to spin_unlock_wait() was required to have all
acquisitions followed by smp_mb__after_unlock_lock() or some such.
But I haven't yet given up on finding a better solution.

							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/