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

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Message-ID: <20131210051910.GI4208@linux.vnet.ibm.com>
Date:	Mon, 9 Dec 2013 21:19:10 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	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, peterz@...radead.org,
	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 Mon, Dec 09, 2013 at 05:32:31PM -0800, Josh Triplett wrote:
> On Mon, Dec 09, 2013 at 05:28:01PM -0800, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> > 
> > Historically, an UNLOCK+LOCK pair executed by one CPU, by one task,
> > or on a given lock variable has implied a full memory barrier.  In a
> > recent LKML thread, the wisdom of this historical approach was called
> > into question: http://www.spinics.net/lists/linux-mm/msg65653.html,
> > in part due to the memory-order complexities of low-handoff-overhead
> > queued locks on x86 systems.
> > 
> > This patch therefore removes this guarantee from the documentation, and
> > further documents how to restore it via a new smp_mb__after_unlock_lock()
> > primitive.
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > Cc: Ingo Molnar <mingo@...hat.com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Oleg Nesterov <oleg@...hat.com>
> > Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> > Cc: Will Deacon <will.deacon@....com>
> > Cc: Tim Chen <tim.c.chen@...ux.intel.com>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Waiman Long <waiman.long@...com>
> > Cc: Andrea Arcangeli <aarcange@...hat.com>
> > Cc: Andi Kleen <andi@...stfloor.org>
> > Cc: Michel Lespinasse <walken@...gle.com>
> > Cc: Davidlohr Bueso <davidlohr.bueso@...com>
> > Cc: Rik van Riel <riel@...hat.com>
> > Cc: Peter Hurley <peter@...leysoftware.com>
> > Cc: "H. Peter Anvin" <hpa@...or.com>
> > Cc: Arnd Bergmann <arnd@...db.de>
> > Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> > ---
> >  Documentation/memory-barriers.txt | 51 +++++++++++++++++++++++++++++++++------
> >  1 file changed, 44 insertions(+), 7 deletions(-)
> > 
> > diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> > index a0763db314ff..efb791d33e5a 100644
> > --- a/Documentation/memory-barriers.txt
> > +++ b/Documentation/memory-barriers.txt
> > @@ -1626,7 +1626,10 @@ for each construct.  These operations all imply certain barriers:
> >       operation has completed.
> >  
> >       Memory operations issued before the LOCK may be completed after the LOCK
> > -     operation has completed.
> > +     operation has completed.  An smp_mb__before_spinlock(), combined
> > +     with a following LOCK, acts as an smp_wmb().  Note the "w",
> > +     this is smp_wmb(), not smp_mb().  The smp_mb__before_spinlock()
> > +     primitive is free on many architectures.
> 
> Gah.  That seems highly error-prone; why isn't that
> "smp_wmb__before_spinlock()"?

I must confess that I wondered that myself.  I didn't create it, I am
just documenting it.

Might be worth a change, though.

							Thanx, Paul

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