tools/memory-model: Model smp_mb__after_unlock_lock()

The kernel documents smp_mb__after_unlock_lock() the following way:

  "Place this after a lock-acquisition primitive to guarantee that
   an UNLOCK+LOCK pair acts 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."

Formalize in LKMM the above guarantee by defining (new) mb-links according
to the law:

  ([M] ; po ; [UL] ; (co | po) ; [LKW] ;
	fencerel(After-unlock-lock) ; [M])

where the component ([UL] ; co ; [LKW]) identifies "UNLOCK+LOCK pairs on
the same lock variable" and the component ([UL] ; po ; [LKW]) identifies
"UNLOCK+LOCK pairs executed by the same CPU".

In particular, the LKMM forbids the following two behaviors (the second
litmus test below is based on:

  Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.html

c.f., Section "Tree RCU Grace Period Memory Ordering Building Blocks"):

C after-unlock-lock-same-cpu

(*
 * Result: Never
 *)

{}

P0(spinlock_t *s, spinlock_t *t, int *x, int *y)
{
	int r0;

	spin_lock(s);
	WRITE_ONCE(*x, 1);
	spin_unlock(s);
	spin_lock(t);
	smp_mb__after_unlock_lock();
	r0 = READ_ONCE(*y);
	spin_unlock(t);
}

P1(int *x, int *y)
{
	int r0;

	WRITE_ONCE(*y, 1);
	smp_mb();
	r0 = READ_ONCE(*x);
}

exists (0:r0=0 /\ 1:r0=0)

C after-unlock-lock-same-lock-variable

(*
 * Result: Never
 *)

{}

P0(spinlock_t *s, int *x, int *y)
{
	int r0;

	spin_lock(s);
	WRITE_ONCE(*x, 1);
	r0 = READ_ONCE(*y);
	spin_unlock(s);
}

P1(spinlock_t *s, int *y, int *z)
{
	int r0;

	spin_lock(s);
	smp_mb__after_unlock_lock();
	WRITE_ONCE(*y, 1);
	r0 = READ_ONCE(*z);
	spin_unlock(s);
}

P2(int *z, int *x)
{
	int r0;

	WRITE_ONCE(*z, 1);
	smp_mb();
	r0 = READ_ONCE(*x);
}

exists (0:r0=0 /\ 1:r0=0 /\ 2:r0=0)

Signed-off-by: Andrea Parri <andrea.parri@amarulasolutions.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
Cc: Akira Yokosawa <akiyks@gmail.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Daniel Lustig <dlustig@nvidia.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Jade Alglave <j.alglave@ucl.ac.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Luc Maranget <luc.maranget@inria.fr>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will.deacon@arm.com>
Cc: linux-arch@vger.kernel.org
Cc: parri.andrea@gmail.com
Link: http://lkml.kernel.org/r/20181203230451.28921-1-paulmck@linux.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/tools/memory-model/linux-kernel.bell b/tools/memory-model/linux-kernel.bell
index b84fb2f67109e03c2e32558599dbf48357d6863e..796513362c0522596f4fd9fdd967257839802ced 100644 (file)
--- a/tools/memory-model/linux-kernel.bell
+++ b/tools/memory-model/linux-kernel.bell
@@ -29,7 +29,8 @@ enum Barriers = 'wmb (*smp_wmb*) ||
                'sync-rcu (*synchronize_rcu*) ||
                'before-atomic (*smp_mb__before_atomic*) ||
                'after-atomic (*smp_mb__after_atomic*) ||
-               'after-spinlock (*smp_mb__after_spinlock*)
+               'after-spinlock (*smp_mb__after_spinlock*) ||
+               'after-unlock-lock (*smp_mb__after_unlock_lock*)
 instructions F[Barriers]
 
 (* Compute matching pairs of nested Rcu-lock and Rcu-unlock *)

diff --git a/tools/memory-model/linux-kernel.cat b/tools/memory-model/linux-kernel.cat
index 882fc33274ac3ce1ca64695e736e7dd755ab7198..8f23c74a96fdca4775bc463b46838c1d57c496ec 100644 (file)
--- a/tools/memory-model/linux-kernel.cat
+++ b/tools/memory-model/linux-kernel.cat
@@ -30,7 +30,9 @@ let wmb = [W] ; fencerel(Wmb) ; [W]
 let mb = ([M] ; fencerel(Mb) ; [M]) |
        ([M] ; fencerel(Before-atomic) ; [RMW] ; po? ; [M]) |
        ([M] ; po? ; [RMW] ; fencerel(After-atomic) ; [M]) |
-       ([M] ; po? ; [LKW] ; fencerel(After-spinlock) ; [M])
+       ([M] ; po? ; [LKW] ; fencerel(After-spinlock) ; [M]) |
+       ([M] ; po ; [UL] ; (co | po) ; [LKW] ;
+               fencerel(After-unlock-lock) ; [M])
 let gp = po ; [Sync-rcu] ; po?
 
 let strong-fence = mb | gp

diff --git a/tools/memory-model/linux-kernel.def b/tools/memory-model/linux-kernel.def
index 6fa3eb28d40b51a4cdc35910dab3f7430337e3d5..b27911cc087d426c49c22e3d3c41cd63e4230d07 100644 (file)
--- a/tools/memory-model/linux-kernel.def
+++ b/tools/memory-model/linux-kernel.def
@@ -23,6 +23,7 @@ smp_wmb() { __fence{wmb}; }
 smp_mb__before_atomic() { __fence{before-atomic}; }
 smp_mb__after_atomic() { __fence{after-atomic}; }
 smp_mb__after_spinlock() { __fence{after-spinlock}; }
+smp_mb__after_unlock_lock() { __fence{after-unlock-lock}; }
 
 // Exchange
 xchg(X,V)  __xchg{mb}(X,V)