[PATCH locking 0/4] locktorture and memory-barriers.txt updates

[PATCH locking 0/4] locktorture and memory-barriers.txt updates

[Paul E. McKenney]

Hello!

This series contains a few memory-barriers.txt updates and a locktorture
cleanup:

1.	Add a disclaimer to memory-barrier.txt, courtesy of
	Peter Zijlstra.

2.	Explicitly state the purpose of memory-barrier.txt, courtesy
	of David Howells.

3.	Explicitly state that ACQUIRE applies to loads and that
	RELEASE applies to stores, courtesy of Will Deacon.

4.	Simplify torture_runnable computation in locktorture, replacing
	a multiline #ifdef with an IS_ENABLED() that fits into an
	existing line.

							Thanx, Paul

------------------------------------------------------------------------

 Documentation/memory-barriers.txt |   39 +++++++++++++++++++++++++++++++++++++-
 kernel/locking/locktorture.c      |    7 ------
 2 files changed, 39 insertions(+), 7 deletions(-)

[PATCH locking 1/4] documentation: Add disclaimer

[Paul E. McKenney]

From: Peter Zijlstra <peterz@infradead.org>

It appears people are reading this document as a requirements list for
building hardware. This is not the intent of this document. Nor is it
particularly suited for this purpose.

The primary purpose of this document is our collective attempt to define
a set of primitives that (hopefully) allow us to write correct code on
the myriad of SMP platforms Linux supports.

Its a definite work in progress as our understanding of these platforms,
and memory ordering in general, progresses.

Nor does being mentioned in this document mean we think its a
particularly good idea; the data dependency barrier required by Alpha
being a prime example. Yes we have it, no you're insane to require it
when building new hardware.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 Documentation/memory-barriers.txt | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index a9454b1c73bd..fb2dd35a823a 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -4,8 +4,24 @@
 
 By: David Howells <dhowells@redhat.com>
     Paul E. McKenney <paulmck@linux.vnet.ibm.com>
+    Will Deacon <will.deacon@arm.com>
+    Peter Zijlstra <peterz@infradead.org>
 
-Contents:
+==========
+DISCLAIMER
+==========
+
+This document is not a specification; it is intentionally (for the sake of
+brevity) and unintentionally (due to being human) incomplete. This document is
+meant as a guide to using the various memory barriers provided by Linux, but
+in case of any doubt (and there are many) please ask.
+
+To repeat, this document is not a specification of what Linux expects from
+hardware.
+
+========
+CONTENTS
+========
 
  (*) Abstract memory access model.
 
-- 
2.5.2

[PATCH locking 2/4] documentation: State purpose of memory-barriers.txt

[Paul E. McKenney]

From: David Howells <dhowells@redhat.com>

There has been some confusion about the purpose of memory-barriers.txt,
so this commit adds a statement of purpose.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 Documentation/memory-barriers.txt | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index fb2dd35a823a..8b11e54238bf 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -19,6 +19,22 @@ in case of any doubt (and there are many) please ask.
 To repeat, this document is not a specification of what Linux expects from
 hardware.
 
+The purpose of this document is twofold:
+
+ (1) to specify the minimum functionality that one can rely on for any
+     particular barrier, and
+
+ (2) to provide a guide as to how to use the barriers that are available.
+
+Note that an architecture can provide more than the minimum requirement
+for any particular barrier, but if the architecure provides less than
+that, that architecture is incorrect.
+
+Note also that it is possible that a barrier may be a no-op for an
+architecture because the way that arch works renders an explicit barrier
+unnecessary in that case.
+
+
 ========
 CONTENTS
 ========
-- 
2.5.2

[PATCH locking 3/4] documentation: ACQUIRE applies to loads, RELEASE applies to stores

[Paul E. McKenney]

From: Will Deacon <will.deacon@arm.com>

For compound atomics performing both a load and a store operation, make
it clear that _acquire and _release variants refer only to the load and
store portions of compound atomic. For example, xchg_acquire is an xchg
operation where the load takes on ACQUIRE semantics.

Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 Documentation/memory-barriers.txt | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 8b11e54238bf..147ae8ec836f 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -498,6 +498,11 @@ And a couple of implicit varieties:
      This means that ACQUIRE acts as a minimal "acquire" operation and
      RELEASE acts as a minimal "release" operation.
 
+A subset of the atomic operations described in atomic_ops.txt have ACQUIRE
+and RELEASE variants in addition to fully-ordered and relaxed (no barrier
+semantics) definitions.  For compound atomics performing both a load and a
+store, ACQUIRE semantics apply only to the load and RELEASE semantics apply
+only to the store portion of the operation.
 
 Memory barriers are only required where there's a possibility of interaction
 between two CPUs or between a CPU and a device.  If it can be guaranteed that
-- 
2.5.2

[PATCH locking 4/4] locktorture: Simplify torture_runnable computation

[Paul E. McKenney]

This commit replaces a #ifdef with IS_ENABLED(), saving five lines.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/locking/locktorture.c | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/kernel/locking/locktorture.c b/kernel/locking/locktorture.c
index d066a50dc87e..f8c5af52a131 100644
--- a/kernel/locking/locktorture.c
+++ b/kernel/locking/locktorture.c
@@ -75,12 +75,7 @@ struct lock_stress_stats {
 	long n_lock_acquired;
 };
 
-#if defined(MODULE)
-#define LOCKTORTURE_RUNNABLE_INIT 1
-#else
-#define LOCKTORTURE_RUNNABLE_INIT 0
-#endif
-int torture_runnable = LOCKTORTURE_RUNNABLE_INIT;
+int torture_runnable = IS_ENABLED(MODULE);
 module_param(torture_runnable, int, 0444);
 MODULE_PARM_DESC(torture_runnable, "Start locktorture at module init");
 
-- 
2.5.2

[tip:locking/core] locking/Documentation: Add disclaimer

[tip-bot for Peter Zijlstra]

Commit-ID:  e7720af5f9ac914577e2b810d5c004cdf395fd82
Gitweb:     http://git.kernel.org/tip/e7720af5f9ac914577e2b810d5c004cdf395fd82
Author:     Peter Zijlstra <peterz@infradead.org>
AuthorDate: Tue, 26 Apr 2016 10:22:05 -0700
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Thu, 28 Apr 2016 10:57:51 +0200

locking/Documentation: Add disclaimer

It appears people are reading this document as a requirements list for
building hardware. This is not the intent of this document. Nor is it
particularly suited for this purpose.

The primary purpose of this document is our collective attempt to define
a set of primitives that (hopefully) allow us to write correct code on
the myriad of SMP platforms Linux supports.

Its a definite work in progress as our understanding of these platforms,
and memory ordering in general, progresses.

Nor does being mentioned in this document mean we think its a
particularly good idea; the data dependency barrier required by Alpha
being a prime example. Yes we have it, no you're insane to require it
when building new hardware.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: corbet@lwn.net
Cc: dave@stgolabs.net
Cc: dhowells@redhat.com
Cc: linux-doc@vger.kernel.org
Cc: will.deacon@arm.com
Link: http://lkml.kernel.org/r/1461691328-5429-1-git-send-email-paulmck@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 Documentation/memory-barriers.txt | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index a9454b1..fb2dd35 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -4,8 +4,24 @@
 
 By: David Howells <dhowells@redhat.com>
     Paul E. McKenney <paulmck@linux.vnet.ibm.com>
+    Will Deacon <will.deacon@arm.com>
+    Peter Zijlstra <peterz@infradead.org>
 
-Contents:
+==========
+DISCLAIMER
+==========
+
+This document is not a specification; it is intentionally (for the sake of
+brevity) and unintentionally (due to being human) incomplete. This document is
+meant as a guide to using the various memory barriers provided by Linux, but
+in case of any doubt (and there are many) please ask.
+
+To repeat, this document is not a specification of what Linux expects from
+hardware.
+
+========
+CONTENTS
+========
 
  (*) Abstract memory access model.
 

[tip:locking/core] locking/Documentation: State purpose of memory-barriers.txt

[tip-bot for David Howells]

Commit-ID:  8d4840e84871847ee1bae56a776907d08a9265f7
Gitweb:     http://git.kernel.org/tip/8d4840e84871847ee1bae56a776907d08a9265f7
Author:     David Howells <dhowells@redhat.com>
AuthorDate: Tue, 26 Apr 2016 10:22:06 -0700
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Thu, 28 Apr 2016 10:57:51 +0200

locking/Documentation: State purpose of memory-barriers.txt

There has been some confusion about the purpose of memory-barriers.txt,
so this commit adds a statement of purpose.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: corbet@lwn.net
Cc: dave@stgolabs.net
Cc: linux-doc@vger.kernel.org
Cc: will.deacon@arm.com
Link: http://lkml.kernel.org/r/1461691328-5429-2-git-send-email-paulmck@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 Documentation/memory-barriers.txt | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index fb2dd35..8b11e54 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -19,6 +19,22 @@ in case of any doubt (and there are many) please ask.
 To repeat, this document is not a specification of what Linux expects from
 hardware.
 
+The purpose of this document is twofold:
+
+ (1) to specify the minimum functionality that one can rely on for any
+     particular barrier, and
+
+ (2) to provide a guide as to how to use the barriers that are available.
+
+Note that an architecture can provide more than the minimum requirement
+for any particular barrier, but if the architecure provides less than
+that, that architecture is incorrect.
+
+Note also that it is possible that a barrier may be a no-op for an
+architecture because the way that arch works renders an explicit barrier
+unnecessary in that case.
+
+
 ========
 CONTENTS
 ========

[tip:locking/core] locking/Documentation: Clarify that ACQUIRE applies to loads, RELEASE applies to stores

[tip-bot for Will Deacon]

Commit-ID:  3cfe2e8bc1cf74d78df6fe5ca3a1e1805472a004
Gitweb:     http://git.kernel.org/tip/3cfe2e8bc1cf74d78df6fe5ca3a1e1805472a004
Author:     Will Deacon <will.deacon@arm.com>
AuthorDate: Tue, 26 Apr 2016 10:22:07 -0700
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Thu, 28 Apr 2016 10:57:51 +0200

locking/Documentation: Clarify that ACQUIRE applies to loads, RELEASE applies to stores

For compound atomics performing both a load and a store operation, make
it clear that _acquire and _release variants refer only to the load and
store portions of compound atomic. For example, xchg_acquire is an xchg
operation where the load takes on ACQUIRE semantics.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: corbet@lwn.net
Cc: dave@stgolabs.net
Cc: dhowells@redhat.com
Cc: linux-doc@vger.kernel.org
Link: http://lkml.kernel.org/r/1461691328-5429-3-git-send-email-paulmck@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 Documentation/memory-barriers.txt | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 8b11e54..147ae8e 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -498,6 +498,11 @@ And a couple of implicit varieties:
      This means that ACQUIRE acts as a minimal "acquire" operation and
      RELEASE acts as a minimal "release" operation.
 
+A subset of the atomic operations described in atomic_ops.txt have ACQUIRE
+and RELEASE variants in addition to fully-ordered and relaxed (no barrier
+semantics) definitions.  For compound atomics performing both a load and a
+store, ACQUIRE semantics apply only to the load and RELEASE semantics apply
+only to the store portion of the operation.
 
 Memory barriers are only required where there's a possibility of interaction
 between two CPUs or between a CPU and a device.  If it can be guaranteed that

[tip:locking/core] lcoking/locktorture: Simplify the torture_runnable computation

[tip-bot for Paul E. McKenney]

Commit-ID:  5db4298133d99b3dfc60d6899ac9df169769c899
Gitweb:     http://git.kernel.org/tip/5db4298133d99b3dfc60d6899ac9df169769c899
Author:     Paul E. McKenney <paulmck@linux.vnet.ibm.com>
AuthorDate: Tue, 26 Apr 2016 10:22:08 -0700
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Thu, 28 Apr 2016 10:57:51 +0200

lcoking/locktorture: Simplify the torture_runnable computation

This commit replaces an #ifdef with IS_ENABLED(), saving five lines.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: corbet@lwn.net
Cc: dave@stgolabs.net
Cc: dhowells@redhat.com
Cc: linux-doc@vger.kernel.org
Cc: will.deacon@arm.com
Link: http://lkml.kernel.org/r/1461691328-5429-4-git-send-email-paulmck@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/locking/locktorture.c | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/kernel/locking/locktorture.c b/kernel/locking/locktorture.c
index d066a50..f8c5af5 100644
--- a/kernel/locking/locktorture.c
+++ b/kernel/locking/locktorture.c
@@ -75,12 +75,7 @@ struct lock_stress_stats {
 	long n_lock_acquired;
 };
 
-#if defined(MODULE)
-#define LOCKTORTURE_RUNNABLE_INIT 1
-#else
-#define LOCKTORTURE_RUNNABLE_INIT 0
-#endif
-int torture_runnable = LOCKTORTURE_RUNNABLE_INIT;
+int torture_runnable = IS_ENABLED(MODULE);
 module_param(torture_runnable, int, 0444);
 MODULE_PARM_DESC(torture_runnable, "Start locktorture at module init");