Allow BKL re-acquire to fail, causing us to re-schedule.

This allows for low-latency BKL contention even with
preemption. Previously, since preemption is disabled
over context switches, re-acquiring the kernel lock when
resuming a process would be non-preemtible.

1 files changed, 19 insertions, 5 deletions

diff --git a/include/linux/smp_lock.h b/include/linux/smp_lock.h
index 99ea247059c1..17ff18b82cd9 100644
--- a/include/linux/smp_lock.h
+++ b/include/linux/smp_lock.h
@@ -9,7 +9,7 @@
 
 #define kernel_locked()		(current->lock_depth >= 0)
 
-extern void __lockfunc get_kernel_lock(void);
+extern int __lockfunc get_kernel_lock(void);
 extern void __lockfunc put_kernel_lock(void);
 
 /*
@@ -20,10 +20,24 @@ extern void __lockfunc put_kernel_lock(void);
 		put_kernel_lock();		\
 } while (0)
 
-#define reacquire_kernel_lock(tsk) do {	\
-	if (unlikely((tsk)->lock_depth >= 0))	\
-		get_kernel_lock();		\
-} while (0)
+/*
+ * Non-SMP kernels will never block on the kernel lock,
+ * so we are better off returning a constant zero from
+ * reacquire_kernel_lock() so that the compiler can see
+ * it at compile-time.
+ */
+#ifdef CONFIG_SMP
+#define return_value_on_smp return
+#else
+#define return_value_on_smp
+#endif
+
+static inline int reacquire_kernel_lock(struct task_struct *task)
+{
+	if (unlikely(task->lock_depth >= 0))
+		return_value_on_smp get_kernel_lock();
+	return 0;
+}
 
 extern void __lockfunc lock_kernel(void)	__acquires(kernel_lock);
 extern void __lockfunc unlock_kernel(void)	__releases(kernel_lock);