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From: "Paul E. McKenney" <paulmck@linux.ibm.com>
To: linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
	mingo@kernel.org
Cc: stern@rowland.harvard.edu, parri.andrea@gmail.com,
	will.deacon@arm.com, 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,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: [PATCH memory-model 4/5] tools/memory-model: Add more LKMM limitations
Date: Wed, 26 Sep 2018 11:29:19 -0700	[thread overview]
Message-ID: <20180926182920.27644-4-paulmck@linux.ibm.com> (raw)
In-Reply-To: <20180926182845.GA24839@linux.ibm.com>

From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

This commit adds more detail about compiler optimizations and
not-yet-modeled Linux-kernel APIs.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Andrea Parri <andrea.parri@amarulasolutions.com>
---
 tools/memory-model/README | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/tools/memory-model/README b/tools/memory-model/README
index ee987ce20aae..acf9077cffaa 100644
--- a/tools/memory-model/README
+++ b/tools/memory-model/README
@@ -171,6 +171,12 @@ The Linux-kernel memory model has the following limitations:
 	particular, the "THE PROGRAM ORDER RELATION: po AND po-loc"
 	and "A WARNING" sections).
 
+	Note that this limitation in turn limits LKMM's ability to
+	accurately model address, control, and data dependencies.
+	For example, if the compiler can deduce the value of some variable
+	carrying a dependency, then the compiler can break that dependency
+	by substituting a constant of that value.
+
 2.	Multiple access sizes for a single variable are not supported,
 	and neither are misaligned or partially overlapping accesses.
 
@@ -190,6 +196,36 @@ The Linux-kernel memory model has the following limitations:
 	However, a substantial amount of support is provided for these
 	operations, as shown in the linux-kernel.def file.
 
+	a.	When rcu_assign_pointer() is passed NULL, the Linux
+		kernel provides no ordering, but LKMM models this
+		case as a store release.
+
+	b.	The "unless" RMW operations are not currently modeled:
+		atomic_long_add_unless(), atomic_add_unless(),
+		atomic_inc_unless_negative(), and
+		atomic_dec_unless_positive().  These can be emulated
+		in litmus tests, for example, by using atomic_cmpxchg().
+
+	c.	The call_rcu() function is not modeled.  It can be
+		emulated in litmus tests by adding another process that
+		invokes synchronize_rcu() and the body of the callback
+		function, with (for example) a release-acquire from
+		the site of the emulated call_rcu() to the beginning
+		of the additional process.
+
+	d.	The rcu_barrier() function is not modeled.  It can be
+		emulated in litmus tests emulating call_rcu() via
+		(for example) a release-acquire from the end of each
+		additional call_rcu() process to the site of the
+		emulated rcu-barrier().
+
+	e.	Sleepable RCU (SRCU) is not modeled.  It can be
+		emulated, but perhaps not simply.
+
+	f.	Reader-writer locking is not modeled.  It can be
+		emulated in litmus tests using atomic read-modify-write
+		operations.
+
 The "herd7" tool has some additional limitations of its own, apart from
 the memory model:
 
@@ -204,3 +240,6 @@ the memory model:
 Some of these limitations may be overcome in the future, but others are
 more likely to be addressed by incorporating the Linux-kernel memory model
 into other tools.
+
+Finally, please note that LKMM is subject to change as hardware, use cases,
+and compilers evolve.
-- 
2.17.1


  parent reply	other threads:[~2018-09-26 18:31 UTC|newest]

Thread overview: 41+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-09-26 18:28 [PATCH memory-model 0/5] Updates to the formal memory model Paul E. McKenney
2018-09-26 18:29 ` [PATCH memory-model 1/5] tools/memory-model: Add litmus-test naming scheme Paul E. McKenney
2018-10-02 10:10   ` [tip:locking/core] " tip-bot for Paul E. McKenney
2018-09-26 18:29 ` [PATCH memory-model 2/5] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire Paul E. McKenney
2018-10-02 10:11   ` [tip:locking/core] " tip-bot for Alan Stern
2021-09-08 11:00     ` Peter Zijlstra
2021-09-08 11:44       ` Peter Zijlstra
2021-09-08 14:42         ` Alan Stern
2021-09-08 15:12           ` Peter Zijlstra
2021-09-08 16:08           ` Linus Torvalds
2021-09-09  7:25             ` Peter Zijlstra
2021-09-09 13:35               ` Will Deacon
2021-09-09 17:02                 ` Linus Torvalds
2021-09-09 18:59                   ` Alan Stern
2021-09-09 17:03                 ` Dan Lustig
2021-09-09 18:00                   ` Paul E. McKenney
2021-09-10 14:20                     ` Boqun Feng
2021-09-10 15:33                       ` Palmer Dabbelt
2021-09-10 16:36                       ` Alan Stern
2021-09-10 17:12                         ` Peter Zijlstra
2021-09-10 17:56                           ` Alan Stern
2021-09-10 17:17                         ` Peter Zijlstra
2021-09-12  0:26                         ` Boqun Feng
2021-09-10  0:01                   ` Boqun Feng
2021-09-10  5:37                     ` Boqun Feng
2021-09-10  9:33                     ` Peter Zijlstra
2021-09-10 10:04                       ` Boqun Feng
2021-09-10 13:48                         ` Dan Lustig
2021-09-10 14:15                           ` Boqun Feng
2021-09-09 17:46                 ` Paul E. McKenney
2021-09-10 11:08                   ` Will Deacon
2021-09-17  3:21                     ` Nicholas Piggin
2021-09-17  5:31                       ` Nicholas Piggin
2021-09-17 14:36                     ` Michael Ellerman
2018-09-26 18:29 ` [PATCH memory-model 3/5] tools/memory-model: Fix a README typo Paul E. McKenney
2018-10-02 10:11   ` [tip:locking/core] " tip-bot for SeongJae Park
2018-09-26 18:29 ` Paul E. McKenney [this message]
2018-10-02 10:12   ` [tip:locking/core] tools/memory-model: Add more LKMM limitations tip-bot for Paul E. McKenney
2018-09-26 18:29 ` [PATCH memory-model 5/5] doc: Replace smp_cond_acquire() with smp_cond_load_acquire() Paul E. McKenney
2018-10-02 10:12   ` [tip:locking/core] locking/memory-barriers: " tip-bot for Andrea Parri
2018-10-02  8:28 ` [PATCH memory-model 0/5] Updates to the formal memory model Ingo Molnar

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