[PATCH RFC v2 04/10] locking/local_lock: add localtry_trylock()

[Vlastimil Babka <vbabka@suse.cz>]

Add a localtry_trylock() variant without _irqsave that will be used in
slab sheaves implementation. Thanks to only disabling preemption and not
irqs, it has a lower overhead. It's not necessary to disable irqs to
avoid a deadlock if the irq context uses trylock and can handle
failures.

Also make the comment of localtry_trylock() more clear, and fix a
compilation failure in localtry_lock_init().

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 include/linux/local_lock.h          | 13 ++++++++++++-
 include/linux/local_lock_internal.h | 31 +++++++++++++++++++++++++++----
 2 files changed, 39 insertions(+), 5 deletions(-)

diff --git a/include/linux/local_lock.h b/include/linux/local_lock.h
index 05c254a5d7d3e6db64d7f81a3a4a10f5a942c29e..1a0bc35839e360d5c8170105849c3883463852f8 100644
--- a/include/linux/local_lock.h
+++ b/include/linux/local_lock.h
@@ -77,6 +77,16 @@
 #define localtry_lock_irqsave(lock, flags)				\
 	__localtry_lock_irqsave(lock, flags)
 
+/**
+ * localtry_trylock - Try to acquire a per CPU local lock.
+ * @lock:	The lock variable
+ *
+ * The function can be used in any context such as NMI or HARDIRQ. Due to
+ * locking constrains it will _always_ fail to acquire the lock in NMI or
+ * HARDIRQ context on PREEMPT_RT.
+ */
+#define localtry_trylock(lock)		__localtry_trylock(lock)
+
 /**
  * localtry_trylock_irqsave - Try to acquire a per CPU local lock, save and disable
  *			      interrupts if acquired
@@ -84,7 +94,8 @@
  * @flags:	Storage for interrupt flags
  *
  * The function can be used in any context such as NMI or HARDIRQ. Due to
- * locking constrains it will _always_ fail to acquire the lock on PREEMPT_RT.
+ * locking constrains it will _always_ fail to acquire the lock in NMI or
+ * HARDIRQ context on PREEMPT_RT.
  */
 #define localtry_trylock_irqsave(lock, flags)				\
 	__localtry_trylock_irqsave(lock, flags)
diff --git a/include/linux/local_lock_internal.h b/include/linux/local_lock_internal.h
index c1369b300777d3ff3700cfd8bd4de8186124f036..67bd13d142fac39bc0f8b2c05eaf81717ff480f9 100644
--- a/include/linux/local_lock_internal.h
+++ b/include/linux/local_lock_internal.h
@@ -137,7 +137,7 @@ do {								\
 #define __localtry_lock_init(lock)				\
 do {								\
 	__local_lock_init(&(lock)->llock);			\
-	WRITE_ONCE(&(lock)->acquired, 0);			\
+	WRITE_ONCE((lock)->acquired, 0);			\
 } while (0)
 
 #define __localtry_lock(lock)					\
@@ -167,6 +167,24 @@ do {								\
 		WRITE_ONCE(lt->acquired, 1);			\
 	} while (0)
 
+#define __localtry_trylock(lock)				\
+	({							\
+		localtry_lock_t *lt;				\
+		bool _ret;					\
+								\
+		preempt_disable();				\
+		lt = this_cpu_ptr(lock);			\
+		if (!READ_ONCE(lt->acquired)) {			\
+			WRITE_ONCE(lt->acquired, 1);		\
+			local_trylock_acquire(&lt->llock);	\
+			_ret = true;				\
+		} else {					\
+			_ret = false;				\
+			preempt_enable();			\
+		}						\
+		_ret;						\
+	})
+
 #define __localtry_trylock_irqsave(lock, flags)			\
 	({							\
 		localtry_lock_t *lt;				\
@@ -275,12 +293,10 @@ do {								\
 #define __localtry_unlock_irq(lock)			__local_unlock(lock)
 #define __localtry_unlock_irqrestore(lock, flags)	__local_unlock_irqrestore(lock, flags)
 
-#define __localtry_trylock_irqsave(lock, flags)			\
+#define __localtry_trylock(lock)				\
 	({							\
 		int __locked;					\
 								\
-		typecheck(unsigned long, flags);		\
-		flags = 0;					\
 		if (in_nmi() | in_hardirq()) {			\
 			__locked = 0;				\
 		} else {					\
@@ -292,4 +308,11 @@ do {								\
 		__locked;					\
 	})
 
+#define __localtry_trylock_irqsave(lock, flags)			\
+	({							\
+		typecheck(unsigned long, flags);		\
+		flags = 0;					\
+		__localtry_trylock(lock);			\
+	})
+
 #endif /* CONFIG_PREEMPT_RT */

-- 
2.48.1