[tip:locking/urgent] locking/core: Remove break_lock field when CONFIG_GENERIC_LOCKBREAK=y

[tip-bot for Will Deacon]

Commit-ID:  d89c70356acf11b7cf47ca5cfcafae5062a85451
Gitweb:     https://git.kernel.org/tip/d89c70356acf11b7cf47ca5cfcafae5062a85451
Author:     Will Deacon <will.deacon@xxxxxxx>
AuthorDate: Tue, 28 Nov 2017 18:42:19 +0000
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 12 Dec 2017 11:24:01 +0100

locking/core: Remove break_lock field when CONFIG_GENERIC_LOCKBREAK=y

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.

Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
Acked-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
Cc: Sebastian Ott <sebott@xxxxxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/1511894539-7988-3-git-send-email-will.deacon@xxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 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 cc0072e..857a72c 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 a391861..3bf2735 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 73548eb..24b4e6f 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 0ebb253..936f3d1 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;							\
 }									\
 									\