Re: [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock_lock()

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Jul 15, 2015 at 11:51:35AM +0100, Will Deacon wrote:
> Hi Paul,
> 
> On Wed, Jul 15, 2015 at 02:38:20AM +0100, Paul E. McKenney wrote:
> > On Tue, Jul 14, 2015 at 12:31:44PM -0700, Paul E. McKenney wrote:
> > > On Tue, Jul 14, 2015 at 03:12:16PM +0100, Will Deacon wrote:
> > > > On Tue, Jul 14, 2015 at 03:00:14PM +0100, Paul E. McKenney wrote:
> > > > > On Tue, Jul 14, 2015 at 01:51:46PM +0100, Will Deacon wrote:
> > > > > > On Tue, Jul 14, 2015 at 01:45:40PM +0100, Paul E. McKenney wrote:
> > > > > > > On Tue, Jul 14, 2015 at 11:04:29AM +0100, Will Deacon wrote:
> > > > > > > > Given that RCU is currently the only user of this barrier, how would you
> > > > > > > > feel about making the barrier local to RCU and not part of the general
> > > > > > > > memory-barrier API?
> > > > > > > 
> > > > > > > In theory, no objection.  Your thought is to leave the definitions where
> > > > > > > they are, mark them as being used only by RCU, and removing mention from
> > > > > > > memory-barriers.txt?  Or did you have something else in mind?
> > > > > > 
> > > > > > Actually, I was thinking of defining them in an RCU header file with an
> > > > > > #ifdef CONFIG_POWERPC for the smb_mb() version. Then you could have a big
> > > > > > comment describing the semantics, or put that in an RCU Documentation file
> > > > > > instead of memory-barriers.txt.
> > > > > > 
> > > > > > That *should* then mean we notice anybody else trying to use the barrier,
> > > > > > because they'd need to send patches to either add something equivalent
> > > > > > or move the definition out again.
> > > > > 
> > > > > My concern with this approach is that someone putting together a new
> > > > > architecture might miss this.  That said, this approach certainly would
> > > > > work for the current architectures.
> > > > 
> > > > I don't think they're any more likely to miss it than with the current
> > > > situation where the generic code defines the macro as a NOP unless you
> > > > explicitly override it.
> > > 
> > > Fair enough...
> > 
> > Like this?
> 
> Precisely! Thanks for cooking the patch -- this lays all my worries to
> rest, so:
> 
>   Acked-by: Will Deacon <will.deacon@arm.com>

Thank you!

> We should continue the discussion with Ben and Michael about whether or
> not the PowerPC locking code can be strengthened, though (making this
> barrier a NOP on all currently supported archs).

Indeed -- if it becomes a NOP on all supported architectures, we might
want to consider just removing it completely.

							Thanx, Paul

> Will
> 
> > commit 695c05d4b9666c50b40a1c022678b5f6e2e3e771
> > Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > Date:   Tue Jul 14 18:35:23 2015 -0700
> > 
> >     rcu,locking: Privatize smp_mb__after_unlock_lock()
> >     
> >     RCU is the only thing that uses smp_mb__after_unlock_lock(), and is
> >     likely the only thing that ever will use it, so this commit makes this
> >     macro private to RCU.
> >     
> >     Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> >     Cc: Will Deacon <will.deacon@arm.com>
> >     Cc: Peter Zijlstra <peterz@infradead.org>
> >     Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> >     Cc: "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>
> > 
> > diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> > index 318523872db5..eafa6a53f72c 100644
> > --- a/Documentation/memory-barriers.txt
> > +++ b/Documentation/memory-barriers.txt
> > @@ -1854,16 +1854,10 @@ RELEASE are to the same lock variable, but only from the perspective of
> >  another CPU not holding that lock.  In short, a ACQUIRE followed by an
> >  RELEASE may -not- be assumed to be a full memory barrier.
> >  
> > -Similarly, the reverse case of a RELEASE followed by an ACQUIRE does not
> > -imply a full memory barrier.  If it is necessary for a RELEASE-ACQUIRE
> > -pair to produce a full barrier, the ACQUIRE can be followed by an
> > -smp_mb__after_unlock_lock() invocation.  This will produce a full barrier
> > -(including transitivity) if either (a) the RELEASE and the ACQUIRE are
> > -executed by the same CPU or task, or (b) the RELEASE and ACQUIRE act on
> > -the same variable.  The smp_mb__after_unlock_lock() primitive is free
> > -on many architectures.  Without smp_mb__after_unlock_lock(), the CPU's
> > -execution of the critical sections corresponding to the RELEASE and the
> > -ACQUIRE can cross, so that:
> > +Similarly, the reverse case of a RELEASE followed by an ACQUIRE does
> > +not imply a full memory barrier.  Therefore, the CPU's execution of the
> > +critical sections corresponding to the RELEASE and the ACQUIRE can cross,
> > +so that:
> >  
> >  	*A = a;
> >  	RELEASE M
> > @@ -1901,29 +1895,6 @@ the RELEASE would simply complete, thereby avoiding the deadlock.
> >  	a sleep-unlock race, but the locking primitive needs to resolve
> >  	such races properly in any case.
> >  
> > -With smp_mb__after_unlock_lock(), the two critical sections cannot overlap.
> > -For example, with the following code, the store to *A will always be
> > -seen by other CPUs before the store to *B:
> > -
> > -	*A = a;
> > -	RELEASE M
> > -	ACQUIRE N
> > -	smp_mb__after_unlock_lock();
> > -	*B = b;
> > -
> > -The operations will always occur in one of the following orders:
> > -
> > -	STORE *A, RELEASE, ACQUIRE, smp_mb__after_unlock_lock(), STORE *B
> > -	STORE *A, ACQUIRE, RELEASE, smp_mb__after_unlock_lock(), STORE *B
> > -	ACQUIRE, STORE *A, RELEASE, smp_mb__after_unlock_lock(), STORE *B
> > -
> > -If the RELEASE and ACQUIRE were instead both operating on the same lock
> > -variable, only the first of these alternatives can occur.  In addition,
> > -the more strongly ordered systems may rule out some of the above orders.
> > -But in any case, as noted earlier, the smp_mb__after_unlock_lock()
> > -ensures that the store to *A will always be seen as happening before
> > -the store to *B.
> > -
> >  Locks and semaphores may not provide any guarantee of ordering on UP compiled
> >  systems, and so cannot be counted on in such a situation to actually achieve
> >  anything at all - especially with respect to I/O accesses - unless combined
> > @@ -2154,40 +2125,6 @@ But it won't see any of:
> >  	*E, *F or *G following RELEASE Q
> >  
> >  
> > -However, if the following occurs:
> > -
> > -	CPU 1				CPU 2
> > -	===============================	===============================
> > -	WRITE_ONCE(*A, a);
> > -	ACQUIRE M		     [1]
> > -	WRITE_ONCE(*B, b);
> > -	WRITE_ONCE(*C, c);
> > -	RELEASE M	     [1]
> > -	WRITE_ONCE(*D, d);		WRITE_ONCE(*E, e);
> > -					ACQUIRE M		     [2]
> > -					smp_mb__after_unlock_lock();
> > -					WRITE_ONCE(*F, f);
> > -					WRITE_ONCE(*G, g);
> > -					RELEASE M	     [2]
> > -					WRITE_ONCE(*H, h);
> > -
> > -CPU 3 might see:
> > -
> > -	*E, ACQUIRE M [1], *C, *B, *A, RELEASE M [1],
> > -		ACQUIRE M [2], *H, *F, *G, RELEASE M [2], *D
> > -
> > -But assuming CPU 1 gets the lock first, CPU 3 won't see any of:
> > -
> > -	*B, *C, *D, *F, *G or *H preceding ACQUIRE M [1]
> > -	*A, *B or *C following RELEASE M [1]
> > -	*F, *G or *H preceding ACQUIRE M [2]
> > -	*A, *B, *C, *E, *F or *G following RELEASE M [2]
> > -
> > -Note that the smp_mb__after_unlock_lock() is critically important
> > -here: Without it CPU 3 might see some of the above orderings.
> > -Without smp_mb__after_unlock_lock(), the accesses are not guaranteed
> > -to be seen in order unless CPU 3 holds lock M.
> > -
> >  
> >  ACQUIRES VS I/O ACCESSES
> >  ------------------------
> > diff --git a/arch/powerpc/include/asm/spinlock.h b/arch/powerpc/include/asm/spinlock.h
> > index 4dbe072eecbe..523673d7583c 100644
> > --- a/arch/powerpc/include/asm/spinlock.h
> > +++ b/arch/powerpc/include/asm/spinlock.h
> > @@ -28,8 +28,6 @@
> >  #include <asm/synch.h>
> >  #include <asm/ppc-opcode.h>
> >  
> > -#define smp_mb__after_unlock_lock()	smp_mb()  /* Full ordering for lock. */
> > -
> >  #ifdef CONFIG_PPC64
> >  /* use 0x800000yy when locked, where yy == CPU number */
> >  #ifdef __BIG_ENDIAN__
> > diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> > index 80d974df0ea0..a9fea7395ba2 100644
> > --- a/kernel/rcu/tree.h
> > +++ b/kernel/rcu/tree.h
> > @@ -645,3 +645,15 @@ static inline void rcu_nocb_q_lengths(struct rcu_data *rdp, long *ql, long *qll)
> >  #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
> >  }
> >  #endif /* #ifdef CONFIG_RCU_TRACE */
> > +
> > +/*
> > + * Place this after a lock-acquisition primitive to guarantee that
> > + * an UNLOCK+LOCK pair act as a full barrier.  This guarantee applies
> > + * if the UNLOCK and LOCK are executed by the same CPU or if the
> > + * UNLOCK and LOCK operate on the same lock variable.
> > + */
> > +#ifdef CONFIG_PPC
> > +#define smp_mb__after_unlock_lock()	smp_mb()  /* Full ordering for lock. */
> > +#else /* #ifdef CONFIG_PPC */
> > +#define smp_mb__after_unlock_lock()	do { } while (0)
> > +#endif /* #else #ifdef CONFIG_PPC */
> > 
>