Merge branch 'locking-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull locking changes from Ingo Molnar:
 "The main changes in this cycle were:

   - pvqspinlock statistics fixes (Davidlohr Bueso)

   - flip atomic_fetch_or() arguments (Peter Zijlstra)

   - locktorture simplification (Paul E.  McKenney)

   - documentation updates (SeongJae Park, David Howells, Davidlohr
     Bueso, Paul E McKenney, Peter Zijlstra, Will Deacon)

   - various fixes"

* 'locking-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  locking/atomics: Flip atomic_fetch_or() arguments
  locking/pvqspinlock: Robustify init_qspinlock_stat()
  locking/pvqspinlock: Avoid double resetting of stats
  lcoking/locktorture: Simplify the torture_runnable computation
  locking/Documentation: Clarify that ACQUIRE applies to loads, RELEASE applies to stores
  locking/Documentation: State purpose of memory-barriers.txt
  locking/Documentation: Add disclaimer
  locking/Documentation/lockdep: Fix spelling mistakes
  locking/lockdep: Deinline register_lock_class(), save 2328 bytes
  locking/locktorture: Fix NULL pointer dereference for cleanup paths
  locking/locktorture: Fix deboosting NULL pointer dereference
  locking/Documentation: Mention smp_cond_acquire()
  locking/Documentation: Insert white spaces consistently
  locking/Documentation: Fix formatting inconsistencies
  locking/Documentation: Add missed subsection in TOC
  locking/Documentation: Fix missed s/lock/acquire renames
  locking/Documentation: Clarify relationship of barrier() to control dependencies

2 files changed, 82 insertions, 39 deletions

diff --git a/Documentation/locking/lockdep-design.txt b/Documentation/locking/lockdep-design.txt
index 5001280e9d82..9de1c158d44c 100644
--- a/Documentation/locking/lockdep-design.txt
+++ b/Documentation/locking/lockdep-design.txt
@@ -97,7 +97,7 @@ between any two lock-classes:
    <hardirq-safe>   ->  <hardirq-unsafe>
    <softirq-safe>   ->  <softirq-unsafe>
 
-The first rule comes from the fact the a hardirq-safe lock could be
+The first rule comes from the fact that a hardirq-safe lock could be
 taken by a hardirq context, interrupting a hardirq-unsafe lock - and
 thus could result in a lock inversion deadlock. Likewise, a softirq-safe
 lock could be taken by an softirq context, interrupting a softirq-unsafe
@@ -220,7 +220,7 @@ calculated, which hash is unique for every lock chain. The hash value,
 when the chain is validated for the first time, is then put into a hash
 table, which hash-table can be checked in a lockfree manner. If the
 locking chain occurs again later on, the hash table tells us that we
-dont have to validate the chain again.
+don't have to validate the chain again.
 
 Troubleshooting:
 ----------------
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 3729cbe60e41..147ae8ec836f 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -4,8 +4,40 @@
 
 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.
+
+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
+========
 
  (*) Abstract memory access model.
 
@@ -31,15 +63,15 @@ Contents:
 
  (*) Implicit kernel memory barriers.
 
-     - Locking functions.
+     - Lock acquisition functions.
      - Interrupt disabling functions.
      - Sleep and wake-up functions.
      - Miscellaneous functions.
 
- (*) Inter-CPU locking barrier effects.
+ (*) Inter-CPU acquiring barrier effects.
 
-     - Locks vs memory accesses.
-     - Locks vs I/O accesses.
+     - Acquires vs memory accesses.
+     - Acquires vs I/O accesses.
 
  (*) Where are memory barriers needed?
 
@@ -61,6 +93,7 @@ Contents:
  (*) The things CPUs get up to.
 
      - And then there's the Alpha.
+     - Virtual Machine Guests.
 
  (*) Example uses.
 
@@ -148,7 +181,7 @@ As a further example, consider this sequence of events:
 
 	CPU 1		CPU 2
 	===============	===============
-	{ A == 1, B == 2, C = 3, P == &A, Q == &C }
+	{ A == 1, B == 2, C == 3, P == &A, Q == &C }
 	B = 4;		Q = P;
 	P = &B		D = *Q;
 
@@ -430,8 +463,9 @@ And a couple of implicit varieties:
      This acts as a one-way permeable barrier.  It guarantees that all memory
      operations after the ACQUIRE operation will appear to happen after the
      ACQUIRE operation with respect to the other components of the system.
-     ACQUIRE operations include LOCK operations and smp_load_acquire()
-     operations.
+     ACQUIRE operations include LOCK operations and both smp_load_acquire()
+     and smp_cond_acquire() operations. The later builds the necessary ACQUIRE
+     semantics from relying on a control dependency and smp_rmb().
 
      Memory operations that occur before an ACQUIRE operation may appear to
      happen after it completes.
@@ -464,6 +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
@@ -517,7 +556,7 @@ following sequence of events:
 
 	CPU 1		      CPU 2
 	===============	      ===============
-	{ A == 1, B == 2, C = 3, P == &A, Q == &C }
+	{ A == 1, B == 2, C == 3, P == &A, Q == &C }
 	B = 4;
 	<write barrier>
 	WRITE_ONCE(P, &B)
@@ -544,7 +583,7 @@ between the address load and the data load:
 
 	CPU 1		      CPU 2
 	===============	      ===============
-	{ A == 1, B == 2, C = 3, P == &A, Q == &C }
+	{ A == 1, B == 2, C == 3, P == &A, Q == &C }
 	B = 4;
 	<write barrier>
 	WRITE_ONCE(P, &B);
@@ -813,9 +852,10 @@ In summary:
       the same variable, then those stores must be ordered, either by
       preceding both of them with smp_mb() or by using smp_store_release()
       to carry out the stores.  Please note that it is -not- sufficient
-      to use barrier() at beginning of each leg of the "if" statement,
-      as optimizing compilers do not necessarily respect barrier()
-      in this case.
+      to use barrier() at beginning of each leg of the "if" statement
+      because, as shown by the example above, optimizing compilers can
+      destroy the control dependency while respecting the letter of the
+      barrier() law.
 
   (*) Control dependencies require at least one run-time conditional
       between the prior load and the subsequent store, and this
@@ -1731,15 +1771,15 @@ The Linux kernel has eight basic CPU memory barriers:
 
 
 All memory barriers except the data dependency barriers imply a compiler
-barrier. Data dependencies do not impose any additional compiler ordering.
+barrier.  Data dependencies do not impose any additional compiler ordering.
 
 Aside: In the case of data dependencies, the compiler would be expected
 to issue the loads in the correct order (eg. `a[b]` would have to load
 the value of b before loading a[b]), however there is no guarantee in
 the C specification that the compiler may not speculate the value of b
 (eg. is equal to 1) and load a before b (eg. tmp = a[1]; if (b != 1)
-tmp = a[b]; ). There is also the problem of a compiler reloading b after
-having loaded a[b], thus having a newer copy of b than a[b]. A consensus
+tmp = a[b]; ).  There is also the problem of a compiler reloading b after
+having loaded a[b], thus having a newer copy of b than a[b].  A consensus
 has not yet been reached about these problems, however the READ_ONCE()
 macro is a good place to start looking.
 
@@ -1794,6 +1834,7 @@ There are some more advanced barrier functions:
 
 
  (*) lockless_dereference();
+
      This can be thought of as a pointer-fetch wrapper around the
      smp_read_barrier_depends() data-dependency barrier.
 
@@ -1858,7 +1899,7 @@ This is a variation on the mandatory write barrier that causes writes to weakly
 ordered I/O regions to be partially ordered.  Its effects may go beyond the
 CPU->Hardware interface and actually affect the hardware at some level.
 
-See the subsection "Locks vs I/O accesses" for more information.
+See the subsection "Acquires vs I/O accesses" for more information.
 
 
 ===============================
@@ -1873,8 +1914,8 @@ provide more substantial guarantees, but these may not be relied upon outside
 of arch specific code.
 
 
-ACQUIRING FUNCTIONS
--------------------
+LOCK ACQUISITION FUNCTIONS
+--------------------------
 
 The Linux kernel has a number of locking constructs:
 
@@ -1895,7 +1936,7 @@ for each construct.  These operations all imply certain barriers:
      Memory operations issued before the ACQUIRE may be completed after
      the ACQUIRE operation has completed.  An smp_mb__before_spinlock(),
      combined with a following ACQUIRE, orders prior stores against
-     subsequent loads and stores. Note that this is weaker than smp_mb()!
+     subsequent loads and stores.  Note that this is weaker than smp_mb()!
      The smp_mb__before_spinlock() primitive is free on many architectures.
 
  (2) RELEASE operation implication:
@@ -2090,9 +2131,9 @@ or:
 	event_indicated = 1;
 	wake_up_process(event_daemon);
 
-A write memory barrier is implied by wake_up() and co. if and only if they wake
-something up.  The barrier occurs before the task state is cleared, and so sits
-between the STORE to indicate the event and the STORE to set TASK_RUNNING:
+A write memory barrier is implied by wake_up() and co.  if and only if they
+wake something up.  The barrier occurs before the task state is cleared, and so
+sits between the STORE to indicate the event and the STORE to set TASK_RUNNING:
 
 	CPU 1				CPU 2
 	===============================	===============================
@@ -2206,7 +2247,7 @@ three CPUs; then should the following sequence of events occur:
 
 Then there is no guarantee as to what order CPU 3 will see the accesses to *A
 through *H occur in, other than the constraints imposed by the separate locks
-on the separate CPUs. It might, for example, see:
+on the separate CPUs.  It might, for example, see:
 
 	*E, ACQUIRE M, ACQUIRE Q, *G, *C, *F, *A, *B, RELEASE Q, *D, *H, RELEASE M
 
@@ -2486,9 +2527,9 @@ The following operations are special locking primitives:
 	clear_bit_unlock();
 	__clear_bit_unlock();
 
-These implement ACQUIRE-class and RELEASE-class operations. These should be used in
-preference to other operations when implementing locking primitives, because
-their implementations can be optimised on many architectures.
+These implement ACQUIRE-class and RELEASE-class operations.  These should be
+used in preference to other operations when implementing locking primitives,
+because their implementations can be optimised on many architectures.
 
 [!] Note that special memory barrier primitives are available for these
 situations because on some CPUs the atomic instructions used imply full memory
@@ -2568,12 +2609,12 @@ explicit barriers are used.
 
 Normally this won't be a problem because the I/O accesses done inside such
 sections will include synchronous load operations on strictly ordered I/O
-registers that form implicit I/O barriers. If this isn't sufficient then an
+registers that form implicit I/O barriers.  If this isn't sufficient then an
 mmiowb() may need to be used explicitly.
 
 
 A similar situation may occur between an interrupt routine and two routines
-running on separate CPUs that communicate with each other. If such a case is
+running on separate CPUs that communicate with each other.  If such a case is
 likely, then interrupt-disabling locks should be used to guarantee ordering.
 
 
@@ -2587,8 +2628,8 @@ functions:
  (*) inX(), outX():
 
      These are intended to talk to I/O space rather than memory space, but
-     that's primarily a CPU-specific concept. The i386 and x86_64 processors do
-     indeed have special I/O space access cycles and instructions, but many
+     that's primarily a CPU-specific concept.  The i386 and x86_64 processors
+     do indeed have special I/O space access cycles and instructions, but many
      CPUs don't have such a concept.
 
      The PCI bus, amongst others, defines an I/O space concept which - on such
@@ -2610,7 +2651,7 @@ functions:
 
      Whether these are guaranteed to be fully ordered and uncombined with
      respect to each other on the issuing CPU depends on the characteristics
-     defined for the memory window through which they're accessing. On later
+     defined for the memory window through which they're accessing.  On later
      i386 architecture machines, for example, this is controlled by way of the
      MTRR registers.
 
@@ -2635,10 +2676,10 @@ functions:
  (*) readX_relaxed(), writeX_relaxed()
 
      These are similar to readX() and writeX(), but provide weaker memory
-     ordering guarantees. Specifically, they do not guarantee ordering with
+     ordering guarantees.  Specifically, they do not guarantee ordering with
      respect to normal memory accesses (e.g. DMA buffers) nor do they guarantee
-     ordering with respect to LOCK or UNLOCK operations. If the latter is
-     required, an mmiowb() barrier can be used. Note that relaxed accesses to
+     ordering with respect to LOCK or UNLOCK operations.  If the latter is
+     required, an mmiowb() barrier can be used.  Note that relaxed accesses to
      the same peripheral are guaranteed to be ordered with respect to each
      other.
 
@@ -3040,8 +3081,9 @@ The Alpha defines the Linux kernel's memory barrier model.
 
 See the subsection on "Cache Coherency" above.
 
+
 VIRTUAL MACHINE GUESTS
--------------------
+----------------------
 
 Guests running within virtual machines might be affected by SMP effects even if
 the guest itself is compiled without SMP support.  This is an artifact of
@@ -3050,7 +3092,7 @@ barriers for this use-case would be possible but is often suboptimal.
 
 To handle this case optimally, low-level virt_mb() etc macros are available.
 These have the same effect as smp_mb() etc when SMP is enabled, but generate
-identical code for SMP and non-SMP systems. For example, virtual machine guests
+identical code for SMP and non-SMP systems.  For example, virtual machine guests
 should use virt_mb() rather than smp_mb() when synchronizing against a
 (possibly SMP) host.
 
@@ -3058,6 +3100,7 @@ These are equivalent to smp_mb() etc counterparts in all other respects,
 in particular, they do not control MMIO effects: to control
 MMIO effects, use mandatory barriers.
 
+
 ============
 EXAMPLE USES
 ============