LKMM uses two relations for talking about UNLOCK+LOCK pairings:

	1) po-unlock-lock-po, which handles UNLOCK+LOCK pairings
	   on the same CPU or immediate lock handovers on the same
	   lock variable

	2) po;[UL];(co|po);[LKW];po, which handles UNLOCK+LOCK pairs
	   literally as described in rcupdate.h#L1002, i.e., even
	   after a sequence of handovers on the same lock variable.

The latter relation is used only once, to provide the guarantee
defined in rcupdate.h#L1002 by smp_mb__after_unlock_lock(), which
makes any UNLOCK+LOCK pair followed by the fence behave like a full
barrier.

This patch drops this use in favor of using po-unlock-lock-po
everywhere, which unifies the way the model talks about UNLOCK+LOCK
pairings.  At first glance this seems to weaken the guarantee given
by LKMM: When considering a long sequence of lock handovers
such as below, where P0 hands the lock to P1, which hands it to P2,
which finally executes such an after_unlock_lock fence, the mb
relation currently links any stores in the critical section of P0
to instructions P2 executes after its fence, but not so after the
patch.

P0(int *x, int *y, spinlock_t *mylock)
{
        spin_lock(mylock);
        WRITE_ONCE(*x, 2);
        spin_unlock(mylock);
        WRITE_ONCE(*y, 1);
}

P1(int *y, int *z, spinlock_t *mylock)
{
        int r0 = READ_ONCE(*y); // reads 1
        spin_lock(mylock);
        spin_unlock(mylock);
        WRITE_ONCE(*z,1);
}

P2(int *z, int *d, spinlock_t *mylock)
{
        int r1 = READ_ONCE(*z); // reads 1
        spin_lock(mylock);
        spin_unlock(mylock);
        smp_mb__after_unlock_lock();
        WRITE_ONCE(*d,1);
}

P3(int *x, int *d)
{
        WRITE_ONCE(*d,2);
        smp_mb();
        WRITE_ONCE(*x,1);
}

exists (1:r0=1 /\ 2:r1=1 /\ x=2 /\ d=2)

Nevertheless, the ordering guarantee given in rcupdate.h is actually
not weakened.  This is because the unlock operations along the
sequence of handovers are A-cumulative fences.  They ensure that any
stores that propagate to the CPU performing the first unlock
operation in the sequence must also propagate to every CPU that
performs a subsequent lock operation in the sequence.  Therefore any
such stores will also be ordered correctly by the fence even if only
the final handover is considered a full barrier.

Indeed this patch does not affect the behaviors allowed by LKMM at
all.  The mb relation is used to define ordering through:
1) mb/.../ppo/hb, where the ordering is subsumed by hb+ where the
   lock-release, rfe, and unlock-acquire orderings each provide hb
2) mb/strong-fence/cumul-fence/prop, where the rfe and A-cumulative
   lock-release orderings simply add more fine-grained cumul-fence
   edges to substitute a single strong-fence edge provided by a long
   lock handover sequence
3) mb/strong-fence/pb and various similar uses in the definition of
   data races, where as discussed above any long handover sequence
   can be turned into a sequence of cumul-fence edges that provide
   the same ordering.
Signed-off-by: Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
Reviewed-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Andrea Parri <parri.andrea@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>