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    SubjectRe: [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@linux.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@linux.vnet.ibm.com>
    > > Cc: Ingo Molnar <mingo@redhat.com>
    > > Cc: Peter Zijlstra <peterz@infradead.org>
    > > Cc: Oleg Nesterov <oleg@redhat.com>
    > > Cc: Linus Torvalds <torvalds@linux-foundation.org>
    > > Cc: Will Deacon <will.deacon@arm.com>
    > > Cc: Tim Chen <tim.c.chen@linux.intel.com>
    > > Cc: Andrew Morton <akpm@linux-foundation.org>
    > > Cc: Thomas Gleixner <tglx@linutronix.de>
    > > Cc: Waiman Long <waiman.long@hp.com>
    > > Cc: Andrea Arcangeli <aarcange@redhat.com>
    > > Cc: Andi Kleen <andi@firstfloor.org>
    > > Cc: Michel Lespinasse <walken@google.com>
    > > Cc: Davidlohr Bueso <davidlohr.bueso@hp.com>
    > > Cc: Rik van Riel <riel@redhat.com>
    > > Cc: Peter Hurley <peter@hurleysoftware.com>
    > > Cc: "H. Peter Anvin" <hpa@zytor.com>
    > > Cc: Arnd Bergmann <arnd@arndb.de>
    > > Cc: Benjamin Herrenschmidt <benh@kernel.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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