linux-kernel - Re: [PATCH v5 tip/core/locking 5/7] Documentation/memory-barriers.txt: Downgrade UNLOCK+LOCK

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: <20131210180511.GZ4208@linux.vnet.ibm.com>
Date:	Tue, 10 Dec 2013 10:05:11 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	niv@...ibm.com, tglx@...utronix.de, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, darren@...art.com,
	fweisbec@...il.com, sbw@....edu, Ingo Molnar <mingo@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Will Deacon <will.deacon@....com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Waiman Long <waiman.long@...com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	"H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH v5 tip/core/locking 5/7]
 Documentation/memory-barriers.txt: Downgrade UNLOCK+LOCK

On Tue, Dec 10, 2013 at 09:43:20AM -0800, Josh Triplett wrote:
> On Tue, Dec 10, 2013 at 06:25:28PM +0100, Peter Zijlstra wrote:
> > On Tue, Dec 10, 2013 at 09:12:47AM -0800, Paul E. McKenney wrote:
> > > > The way I read the above it says that you need
> > > > smp_mb__after_unlock_lock() when the UNLOCK and LOCK are on the same
> > > > variable. That doesn't make sense, I thought that was the one case we
> > > > all agreed on it would indeed be a full barrier without extra trickery.
> > > 
> > > On x86, sure, but smp_mb__after_unlock_lock() is nothingness on x86
> > > anyway.  Other architectures might benefit from requiring that the
> > > smp_mb__after_unlock_lock() be used in this case.
> > 
> > Confused, UNLOCK X, LOCK X, must always be fully serializing. That's the
> > entire purpose of the thing.
> > 
> > The only place you can go play games (and clearly we are going there) is
> > when the UNLOCK and LOCK are on different variables.
> 
> That would certainly be a good assumption to preserve, and it would
> eliminate most of the need for smp_mb__after_unlock_lock().

Perhaps RCU is an outlier, but most of the places where I added
smp_mb__after_unlock_lock() had an unlock of one rcu_node structure's
->lock followed by a lock of another rcu_node structure's ->lock.

							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/