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" <[email protected]>
> >
> > 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 <[email protected]>
> > Cc: Ingo Molnar <[email protected]>
> > Cc: Peter Zijlstra <[email protected]>
> > Cc: Oleg Nesterov <[email protected]>
> > Cc: Linus Torvalds <[email protected]>
> > Cc: Will Deacon <[email protected]>
> > Cc: Tim Chen <[email protected]>
> > Cc: Andrew Morton <[email protected]>
> > Cc: Thomas Gleixner <[email protected]>
> > Cc: Waiman Long <[email protected]>
> > Cc: Andrea Arcangeli <[email protected]>
> > Cc: Andi Kleen <[email protected]>
> > Cc: Michel Lespinasse <[email protected]>
> > Cc: Davidlohr Bueso <[email protected]>
> > Cc: Rik van Riel <[email protected]>
> > Cc: Peter Hurley <[email protected]>
> > Cc: "H. Peter Anvin" <[email protected]>
> > Cc: Arnd Bergmann <[email protected]>
> > Cc: Benjamin Herrenschmidt <[email protected]>
> > ---
> > 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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