[PATCH 2/2] locking/core: Remove break_lock field when CONFIG_GENERIC_LOCKBREAK=y

[Will Deacon]

When CONFIG_GENERIC_LOCKBEAK=y, locking structures grow an extra int break_lock
field which is used to implement raw_spin_is_contended() by setting the field
to 1 when waiting on a lock and clearing it to zero when holding a lock.
However, there are a few problems with this approach:

  - There is a write-write race between a CPU successfully taking the lock
    (and subsequently writing break_lock = 0) and a waiter waiting on
    the lock (and subsequently writing break_lock = 1). This could result
    in a contended lock being reported as uncontended and vice-versa.

  - On machines with store buffers, nothing guarantees that the writes
    to break_lock are visible to other CPUs at any particular time.

  - READ_ONCE/WRITE_ONCE are not used, so the field is potentially
    susceptible to harmful compiler optimisations,

Consequently, the usefulness of this field is unclear and we'd be better off
removing it and allowing architectures to implement raw_spin_is_contended() by
providing a definition of arch_spin_is_contended(), as they can when
CONFIG_GENERIC_LOCKBREAK=n.

Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
Cc: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Sebastian Ott <sebott@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
---
 include/linux/rwlock_types.h   | 3 ---
 include/linux/spinlock.h       | 5 -----
 include/linux/spinlock_types.h | 3 ---
 kernel/locking/spinlock.c      | 9 +--------
 4 files changed, 1 insertion(+), 19 deletions(-)

diff --git a/include/linux/rwlock_types.h b/include/linux/rwlock_types.h
index cc0072e93e36..857a72ceb794 100644
--- a/include/linux/rwlock_types.h
+++ b/include/linux/rwlock_types.h
@@ -10,9 +10,6 @@
  */
 typedef struct {
 	arch_rwlock_t raw_lock;
-#ifdef CONFIG_GENERIC_LOCKBREAK
-	unsigned int break_lock;
-#endif
 #ifdef CONFIG_DEBUG_SPINLOCK
 	unsigned int magic, owner_cpu;
 	void *owner;
diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h
index a39186194cd6..3bf273538840 100644
--- a/include/linux/spinlock.h
+++ b/include/linux/spinlock.h
@@ -107,16 +107,11 @@ do {								\
 
 #define raw_spin_is_locked(lock)	arch_spin_is_locked(&(lock)->raw_lock)
 
-#ifdef CONFIG_GENERIC_LOCKBREAK
-#define raw_spin_is_contended(lock) ((lock)->break_lock)
-#else
-
 #ifdef arch_spin_is_contended
 #define raw_spin_is_contended(lock)	arch_spin_is_contended(&(lock)->raw_lock)
 #else
 #define raw_spin_is_contended(lock)	(((void)(lock), 0))
 #endif /*arch_spin_is_contended*/
-#endif
 
 /*
  * This barrier must provide two things:
diff --git a/include/linux/spinlock_types.h b/include/linux/spinlock_types.h
index 73548eb13a5d..24b4e6f2c1a2 100644
--- a/include/linux/spinlock_types.h
+++ b/include/linux/spinlock_types.h
@@ -19,9 +19,6 @@
 
 typedef struct raw_spinlock {
 	arch_spinlock_t raw_lock;
-#ifdef CONFIG_GENERIC_LOCKBREAK
-	unsigned int break_lock;
-#endif
 #ifdef CONFIG_DEBUG_SPINLOCK
 	unsigned int magic, owner_cpu;
 	void *owner;
diff --git a/kernel/locking/spinlock.c b/kernel/locking/spinlock.c
index 0ebb253e2199..936f3d14dd6b 100644
--- a/kernel/locking/spinlock.c
+++ b/kernel/locking/spinlock.c
@@ -66,12 +66,8 @@ void __lockfunc __raw_##op##_lock(locktype##_t *lock)			\
 			break;						\
 		preempt_enable();					\
 									\
-		if (!(lock)->break_lock)				\
-			(lock)->break_lock = 1;				\
-									\
 		arch_##op##_relax(&lock->raw_lock);			\
 	}								\
-	(lock)->break_lock = 0;						\
 }									\
 									\
 unsigned long __lockfunc __raw_##op##_lock_irqsave(locktype##_t *lock)	\
@@ -86,12 +82,9 @@ unsigned long __lockfunc __raw_##op##_lock_irqsave(locktype##_t *lock)	\
 		local_irq_restore(flags);				\
 		preempt_enable();					\
 									\
-		if (!(lock)->break_lock)				\
-			(lock)->break_lock = 1;				\
-									\
 		arch_##op##_relax(&lock->raw_lock);			\
 	}								\
-	(lock)->break_lock = 0;						\
+									\
 	return flags;							\
 }									\
 									\
-- 
2.1.4