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Subject[tip:locking/core] tools/memory-model: Model smp_mb__after_unlock_lock()
Commit-ID:  5b735eb1ce481b2f1674a47c0995944b1cb6f5d5
Gitweb: https://git.kernel.org/tip/5b735eb1ce481b2f1674a47c0995944b1cb6f5d5
Author: Andrea Parri <andrea.parri@amarulasolutions.com>
AuthorDate: Mon, 3 Dec 2018 15:04:49 -0800
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 21 Jan 2019 11:06:55 +0100

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>
---
tools/memory-model/linux-kernel.bell | 3 ++-
tools/memory-model/linux-kernel.cat | 4 +++-
tools/memory-model/linux-kernel.def | 1 +
3 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/tools/memory-model/linux-kernel.bell b/tools/memory-model/linux-kernel.bell
index b84fb2f67109..796513362c05 100644
--- 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 882fc33274ac..8f23c74a96fd 100644
--- 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 6fa3eb28d40b..b27911cc087d 100644
--- 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)
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