From: Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
To: paulmck@kernel.org
Cc: stern@rowland.harvard.edu, parri.andrea@gmail.com,
will@kernel.org, peterz@infradead.org, boqun.feng@gmail.com,
npiggin@gmail.com, dhowells@redhat.com, j.alglave@ucl.ac.uk,
luc.maranget@inria.fr, akiyks@gmail.com, dlustig@nvidia.com,
joel@joelfernandes.org, urezki@gmail.com,
quic_neeraju@quicinc.com, frederic@kernel.org,
linux-kernel@vger.kernel.org,
Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
Subject: [PATCH v2 1/2] tools/memory-model: Unify UNLOCK+LOCK pairings to po-unlock-lock-po
Date: Thu, 26 Jan 2023 14:46:03 +0100 [thread overview]
Message-ID: <20230126134604.2160-2-jonas.oberhauser@huaweicloud.com> (raw)
In-Reply-To: <20230126134604.2160-1-jonas.oberhauser@huaweicloud.com>
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>
---
tools/memory-model/linux-kernel.cat | 15 +++++++++++++--
1 file changed, 13 insertions(+), 2 deletions(-)
diff --git a/tools/memory-model/linux-kernel.cat b/tools/memory-model/linux-kernel.cat
index 07f884f9b2bf..6e531457bb73 100644
--- a/tools/memory-model/linux-kernel.cat
+++ b/tools/memory-model/linux-kernel.cat
@@ -37,8 +37,19 @@ 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 ; [UL] ; (co | po) ; [LKW] ;
- fencerel(After-unlock-lock) ; [M])
+(*
+ * Note: The po-unlock-lock-po relation only passes the lock to the direct
+ * successor, perhaps giving the impression that the ordering of the
+ * smp_mb__after_unlock_lock() fence only affects a single lock handover.
+ * However, in a longer sequence of lock handovers, the implicit
+ * A-cumulative release fences of lock-release ensure that any stores that
+ * propagate to one of the involved CPUs before it hands over the lock to
+ * the next CPU will also propagate to the final CPU handing over the lock
+ * to the CPU that executes the fence. Therefore, all those stores are
+ * also affected by the fence.
+ *)
+ ([M] ; po-unlock-lock-po ;
+ [After-unlock-lock] ; po ; [M])
let gp = po ; [Sync-rcu | Sync-srcu] ; po?
let strong-fence = mb | gp
--
2.17.1
next prev parent reply other threads:[~2023-01-26 13:42 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-26 13:46 [PATCH v2 0/2] Streamlining treatment of smp_mb__after_unlock_lock Jonas Oberhauser
2023-01-26 13:46 ` Jonas Oberhauser [this message]
2023-01-26 16:36 ` [PATCH v2 1/2] tools/memory-model: Unify UNLOCK+LOCK pairings to po-unlock-lock-po Alan Stern
2023-01-26 20:08 ` Paul E. McKenney
2023-01-26 23:21 ` Paul E. McKenney
2023-01-27 13:18 ` Jonas Oberhauser
2023-01-27 15:13 ` Paul E. McKenney
2023-01-27 15:57 ` Jonas Oberhauser
2023-01-27 16:48 ` Paul E. McKenney
2023-01-26 13:46 ` [PATCH v2 2/2] tools/memory-model: Make ppo a subrelation of po Jonas Oberhauser
2023-01-26 16:36 ` Alan Stern
2023-01-27 14:31 ` Jonas Oberhauser
2023-01-28 19:56 ` Alan Stern
2023-01-28 22:14 ` Andrea Parri
2023-01-28 22:21 ` Andrea Parri
2023-01-28 22:59 ` Alan Stern
2023-01-29 5:17 ` Paul E. McKenney
2023-01-29 16:03 ` Alan Stern
2023-01-29 16:21 ` Paul E. McKenney
2023-01-29 17:28 ` Andrea Parri
2023-01-29 18:44 ` Paul E. McKenney
2023-01-29 21:43 ` Boqun Feng
2023-01-29 23:09 ` Paul E. McKenney
2023-01-30 2:18 ` Alan Stern
2023-01-30 4:43 ` Paul E. McKenney
2023-01-29 19:17 ` Paul E. McKenney
2023-01-29 17:11 ` Andrea Parri
2023-01-29 22:10 ` Alan Stern
2023-01-29 22:19 ` Jonas Oberhauser
2023-01-30 2:39 ` Alan Stern
2023-01-30 4:36 ` Paul E. McKenney
2023-01-30 16:47 ` Alan Stern
2023-01-30 16:50 ` Paul E. McKenney
2023-01-31 13:56 ` Jonas Oberhauser
2023-01-31 15:06 ` Alan Stern
2023-01-31 15:33 ` Jonas Oberhauser
2023-01-31 16:55 ` Alan Stern
2023-02-01 10:37 ` Jonas Oberhauser
2023-01-30 4:46 ` Paul E. McKenney
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