4 files changed, 95 insertions, 81 deletions

diff --git a/arch/mn10300/Kconfig b/arch/mn10300/Kconfig
index 6a6409adc564..e856218da90d 100644
--- a/arch/mn10300/Kconfig
+++ b/arch/mn10300/Kconfig
@@ -186,17 +186,6 @@ config PREEMPT
           Say Y here if you are building a kernel for a desktop, embedded
           or real-time system.  Say N if you are unsure.
 
-config PREEMPT_BKL
-        bool "Preempt The Big Kernel Lock"
-        depends on PREEMPT
-        default y
-        help
-          This option reduces the latency of the kernel by making the
-          big kernel lock preemptible.
-
-          Say Y here if you are building a kernel for a desktop system.
-          Say N if you are unsure.
-
 config MN10300_CURRENT_IN_E2
         bool "Hold current task address in E2 register"
         default y
diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h
index 897f723bd222..181006cc94a0 100644
--- a/include/linux/hardirq.h
+++ b/include/linux/hardirq.h
@@ -72,6 +72,14 @@
 #define in_softirq()            (softirq_count())
 #define in_interrupt()          (irq_count())
 
+#if defined(CONFIG_PREEMPT)
+# define PREEMPT_INATOMIC_BASE kernel_locked()
+# define PREEMPT_CHECK_OFFSET 1
+#else
+# define PREEMPT_INATOMIC_BASE 0
+# define PREEMPT_CHECK_OFFSET 0
+#endif
+
 /*
  * Are we running in atomic context?  WARNING: this macro cannot
  * always detect atomic context; in particular, it cannot know about
@@ -79,17 +87,11 @@
  * used in the general case to determine whether sleeping is possible.
  * Do not use in_atomic() in driver code.
  */
-#define in_atomic()             ((preempt_count() & ~PREEMPT_ACTIVE) != 0)
-
-#ifdef CONFIG_PREEMPT
-# define PREEMPT_CHECK_OFFSET 1
-#else
-# define PREEMPT_CHECK_OFFSET 0
-#endif
+#define in_atomic()     ((preempt_count() & ~PREEMPT_ACTIVE) != PREEMPT_INATOMIC_BASE)
 
 /*
  * Check whether we were atomic before we did preempt_disable():
- * (used by the scheduler)
+ * (used by the scheduler, *after* releasing the kernel lock)
  */
 #define in_atomic_preempt_off() \
                 ((preempt_count() & ~PREEMPT_ACTIVE) != PREEMPT_CHECK_OFFSET)
diff --git a/kernel/sched.c b/kernel/sched.c
index 58fb8af15776..c51b6565e07c 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4567,8 +4567,6 @@ EXPORT_SYMBOL(schedule);
 asmlinkage void __sched preempt_schedule(void)
 {
         struct thread_info *ti = current_thread_info();
-        struct task_struct *task = current;
-        int saved_lock_depth;
 
         /*
          * If there is a non-zero preempt_count or interrupts are disabled,
@@ -4579,16 +4577,7 @@ asmlinkage void __sched preempt_schedule(void)
 
         do {
                 add_preempt_count(PREEMPT_ACTIVE);
-
-                /*
-                 * We keep the big kernel semaphore locked, but we
-                 * clear ->lock_depth so that schedule() doesnt
-                 * auto-release the semaphore:
-                 */
-                saved_lock_depth = task->lock_depth;
-                task->lock_depth = -1;
                 schedule();
-                task->lock_depth = saved_lock_depth;
                 sub_preempt_count(PREEMPT_ACTIVE);
 
                 /*
@@ -4609,26 +4598,15 @@ EXPORT_SYMBOL(preempt_schedule);
 asmlinkage void __sched preempt_schedule_irq(void)
 {
         struct thread_info *ti = current_thread_info();
-        struct task_struct *task = current;
-        int saved_lock_depth;
 
         /* Catch callers which need to be fixed */
         BUG_ON(ti->preempt_count || !irqs_disabled());
 
         do {
                 add_preempt_count(PREEMPT_ACTIVE);
-
-                /*
-                 * We keep the big kernel semaphore locked, but we
-                 * clear ->lock_depth so that schedule() doesnt
-                 * auto-release the semaphore:
-                 */
-                saved_lock_depth = task->lock_depth;
-                task->lock_depth = -1;
                 local_irq_enable();
                 schedule();
                 local_irq_disable();
-                task->lock_depth = saved_lock_depth;
                 sub_preempt_count(PREEMPT_ACTIVE);
 
                 /*
@@ -5853,8 +5831,11 @@ void __cpuinit init_idle(struct task_struct *idle, int cpu)
         spin_unlock_irqrestore(&rq->lock, flags);
 
         /* Set the preempt count _outside_ the spinlocks! */
+#if defined(CONFIG_PREEMPT)
+        task_thread_info(idle)->preempt_count = (idle->lock_depth >= 0);
+#else
         task_thread_info(idle)->preempt_count = 0;
-
+#endif
         /*
          * The idle tasks have their own, simple scheduling class:
          */
diff --git a/lib/kernel_lock.c b/lib/kernel_lock.c
index cd3e82530b03..01a3c22c1b5a 100644
--- a/lib/kernel_lock.c
+++ b/lib/kernel_lock.c
@@ -11,79 +11,121 @@
 #include <linux/semaphore.h>
 
 /*
- * The 'big kernel semaphore'
+ * The 'big kernel lock'
  *
- * This mutex is taken and released recursively by lock_kernel()
+ * This spinlock is taken and released recursively by lock_kernel()
  * and unlock_kernel().  It is transparently dropped and reacquired
  * over schedule().  It is used to protect legacy code that hasn't
  * been migrated to a proper locking design yet.
  *
- * Note: code locked by this semaphore will only be serialized against
- * other code using the same locking facility. The code guarantees that
- * the task remains on the same CPU.
- *
  * Don't use in new code.
  */
-static DECLARE_MUTEX(kernel_sem);
+static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(kernel_flag);
+
 
 /*
- * Re-acquire the kernel semaphore.
+ * Acquire/release the underlying lock from the scheduler.
  *
- * This function is called with preemption off.
+ * This is called with preemption disabled, and should
+ * return an error value if it cannot get the lock and
+ * TIF_NEED_RESCHED gets set.
  *
- * We are executing in schedule() so the code must be extremely careful
- * about recursion, both due to the down() and due to the enabling of
- * preemption. schedule() will re-check the preemption flag after
- * reacquiring the semaphore.
+ * If it successfully gets the lock, it should increment
+ * the preemption count like any spinlock does.
+ *
+ * (This works on UP too - _raw_spin_trylock will never
+ * return false in that case)
  */
 int __lockfunc __reacquire_kernel_lock(void)
 {
-        struct task_struct *task = current;
-        int saved_lock_depth = task->lock_depth;
-
-        BUG_ON(saved_lock_depth < 0);
-
-        task->lock_depth = -1;
-        preempt_enable_no_resched();
-
-        down(&kernel_sem);
-
+        while (!_raw_spin_trylock(&kernel_flag)) {
+                if (test_thread_flag(TIF_NEED_RESCHED))
+                        return -EAGAIN;
+                cpu_relax();
+        }
         preempt_disable();
-        task->lock_depth = saved_lock_depth;
-
         return 0;
 }
 
 void __lockfunc __release_kernel_lock(void)
 {
-        up(&kernel_sem);
+        _raw_spin_unlock(&kernel_flag);
+        preempt_enable_no_resched();
 }
 
 /*
- * Getting the big kernel semaphore.
+ * These are the BKL spinlocks - we try to be polite about preemption.
+ * If SMP is not on (ie UP preemption), this all goes away because the
+ * _raw_spin_trylock() will always succeed.
  */
-void __lockfunc lock_kernel(void)
+#ifdef CONFIG_PREEMPT
+static inline void __lock_kernel(void)
 {
-        struct task_struct *task = current;
-        int depth = task->lock_depth + 1;
+        preempt_disable();
+        if (unlikely(!_raw_spin_trylock(&kernel_flag))) {
+                /*
+                 * If preemption was disabled even before this
+                 * was called, there's nothing we can be polite
+                 * about - just spin.
+                 */
+                if (preempt_count() > 1) {
+                        _raw_spin_lock(&kernel_flag);
+                        return;
+                }
 
-        if (likely(!depth))
                 /*
-                 * No recursion worries - we set up lock_depth _after_
+                 * Otherwise, let's wait for the kernel lock
+                 * with preemption enabled..
                  */
-                down(&kernel_sem);
+                do {
+                        preempt_enable();
+                        while (spin_is_locked(&kernel_flag))
+                                cpu_relax();
+                        preempt_disable();
+                } while (!_raw_spin_trylock(&kernel_flag));
+        }
+}
 
-        task->lock_depth = depth;
+#else
+
+/*
+ * Non-preemption case - just get the spinlock
+ */
+static inline void __lock_kernel(void)
+{
+        _raw_spin_lock(&kernel_flag);
 }
+#endif
 
-void __lockfunc unlock_kernel(void)
+static inline void __unlock_kernel(void)
 {
-        struct task_struct *task = current;
+        /*
+         * the BKL is not covered by lockdep, so we open-code the
+         * unlocking sequence (and thus avoid the dep-chain ops):
+         */
+        _raw_spin_unlock(&kernel_flag);
+        preempt_enable();
+}
 
-        BUG_ON(task->lock_depth < 0);
+/*
+ * Getting the big kernel lock.
+ *
+ * This cannot happen asynchronously, so we only need to
+ * worry about other CPU's.
+ */
+void __lockfunc lock_kernel(void)
+{
+        int depth = current->lock_depth+1;
+        if (likely(!depth))
+                __lock_kernel();
+        current->lock_depth = depth;
+}
 
-        if (likely(--task->lock_depth < 0))
-                up(&kernel_sem);
+void __lockfunc unlock_kernel(void)
+{
+        BUG_ON(current->lock_depth < 0);
+        if (likely(--current->lock_depth < 0))
+                __unlock_kernel();
 }
 
 EXPORT_SYMBOL(lock_kernel);