locking/barriers: Replace smp_cond_acquire() with smp_cond_load_acquire()

This new form allows using hardware assisted waiting.

Some hardware (ARM64 and x86) allow monitoring an address for changes,
so by providing a pointer we can use this to replace the cpu_relax()
with hardware optimized methods in the future.

Requested-by: Will Deacon <will.deacon@arm.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/compiler.h

@@ -305,21 +305,34 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
 })
 
 /**
- * smp_cond_acquire() - Spin wait for cond with ACQUIRE ordering
+ * smp_cond_load_acquire() - (Spin) wait for cond with ACQUIRE ordering
+ * @ptr: pointer to the variable to wait on
  * @cond: boolean expression to wait for
  *
  * Equivalent to using smp_load_acquire() on the condition variable but employs
  * the control dependency of the wait to reduce the barrier on many platforms.
  *
+ * Due to C lacking lambda expressions we load the value of *ptr into a
+ * pre-named variable @VAL to be used in @cond.
+ *
  * The control dependency provides a LOAD->STORE order, the additional RMB
  * provides LOAD->LOAD order, together they provide LOAD->{LOAD,STORE} order,
  * aka. ACQUIRE.
  */
-#define smp_cond_acquire(cond)	do {		\
-	while (!(cond))				\
-		cpu_relax();			\
-	smp_rmb(); /* ctrl + rmb := acquire */	\
-} while (0)
+#ifndef smp_cond_load_acquire
+#define smp_cond_load_acquire(ptr, cond_expr) ({		\
+	typeof(ptr) __PTR = (ptr);				\
+	typeof(*ptr) VAL;					\
+	for (;;) {						\
+		VAL = READ_ONCE(*__PTR);			\
+		if (cond_expr)					\
+			break;					\
+		cpu_relax();					\
+	}							\
+	smp_rmb(); /* ctrl + rmb := acquire */			\
+	VAL;							\
+})
+#endif
 
 #endif /* __KERNEL__ */