locking/local_lock: Introduce localtry_lock_t

In !PREEMPT_RT local_lock_irqsave() disables interrupts to protect
critical section, but it doesn't prevent NMI, so the fully reentrant
code cannot use local_lock_irqsave() for exclusive access.

Introduce localtry_lock_t and localtry_lock_irqsave() that
disables interrupts and sets acquired=1, so localtry_lock_irqsave()
from NMI attempting to acquire the same lock will return false.

In PREEMPT_RT local_lock_irqsave() maps to preemptible spin_lock().
Map localtry_lock_irqsave() to preemptible spin_trylock().
When in hard IRQ or NMI return false right away, since
spin_trylock() is not safe due to explicit locking in the underneath
rt_spin_trylock() implementation. Removing this explicit locking and
attempting only "trylock" is undesired due to PI implications.

Note there is no need to use local_inc for acquired variable,
since it's a percpu variable with strict nesting scopes.

Acked-by: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Link: https://lore.kernel.org/r/20250222024427.30294-2-alexei.starovoitov@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/local_lock.h

@@ -51,6 +51,76 @@
 #define local_unlock_irqrestore(lock, flags)			\
 	__local_unlock_irqrestore(lock, flags)
 
+/**
+ * localtry_lock_init - Runtime initialize a lock instance
+ */
+#define localtry_lock_init(lock)		__localtry_lock_init(lock)
+
+/**
+ * localtry_lock - Acquire a per CPU local lock
+ * @lock:	The lock variable
+ */
+#define localtry_lock(lock)		__localtry_lock(lock)
+
+/**
+ * localtry_lock_irq - Acquire a per CPU local lock and disable interrupts
+ * @lock:	The lock variable
+ */
+#define localtry_lock_irq(lock)		__localtry_lock_irq(lock)
+
+/**
+ * localtry_lock_irqsave - Acquire a per CPU local lock, save and disable
+ *			 interrupts
+ * @lock:	The lock variable
+ * @flags:	Storage for interrupt flags
+ */
+#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
+ * @lock:	The lock variable
+ * @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 in NMI or
+ * HARDIRQ context on PREEMPT_RT.
+ */
+#define localtry_trylock_irqsave(lock, flags)				\
+	__localtry_trylock_irqsave(lock, flags)
+
+/**
+ * local_unlock - Release a per CPU local lock
+ * @lock:	The lock variable
+ */
+#define localtry_unlock(lock)		__localtry_unlock(lock)
+
+/**
+ * local_unlock_irq - Release a per CPU local lock and enable interrupts
+ * @lock:	The lock variable
+ */
+#define localtry_unlock_irq(lock)		__localtry_unlock_irq(lock)
+
+/**
+ * localtry_unlock_irqrestore - Release a per CPU local lock and restore
+ *			      interrupt flags
+ * @lock:	The lock variable
+ * @flags:      Interrupt flags to restore
+ */
+#define localtry_unlock_irqrestore(lock, flags)			\
+	__localtry_unlock_irqrestore(lock, flags)
+
 DEFINE_GUARD(local_lock, local_lock_t __percpu*,
 	     local_lock(_T),
 	     local_unlock(_T))

include/linux/local_lock_internal.h

@@ -15,6 +15,11 @@ typedef struct {
 #endif
 } local_lock_t;
 
+typedef struct {
+	local_lock_t	llock;
+	unsigned int	acquired;
+} localtry_lock_t;
+
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 # define LOCAL_LOCK_DEBUG_INIT(lockname)		\
 	.dep_map = {					\
@@ -31,6 +36,13 @@ static inline void local_lock_acquire(local_lock_t *l)
 	l->owner = current;
 }
 
+static inline void local_trylock_acquire(local_lock_t *l)
+{
+	lock_map_acquire_try(&l->dep_map);
+	DEBUG_LOCKS_WARN_ON(l->owner);
+	l->owner = current;
+}
+
 static inline void local_lock_release(local_lock_t *l)
 {
 	DEBUG_LOCKS_WARN_ON(l->owner != current);
@@ -45,11 +57,13 @@ static inline void local_lock_debug_init(local_lock_t *l)
 #else /* CONFIG_DEBUG_LOCK_ALLOC */
 # define LOCAL_LOCK_DEBUG_INIT(lockname)
 static inline void local_lock_acquire(local_lock_t *l) { }
+static inline void local_trylock_acquire(local_lock_t *l) { }
 static inline void local_lock_release(local_lock_t *l) { }
 static inline void local_lock_debug_init(local_lock_t *l) { }
 #endif /* !CONFIG_DEBUG_LOCK_ALLOC */
 
 #define INIT_LOCAL_LOCK(lockname)	{ LOCAL_LOCK_DEBUG_INIT(lockname) }
+#define INIT_LOCALTRY_LOCK(lockname)	{ .llock = { LOCAL_LOCK_DEBUG_INIT(lockname.llock) }}
 
 #define __local_lock_init(lock)					\
 do {								\
@@ -118,6 +132,104 @@ do {								\
 #define __local_unlock_nested_bh(lock)				\
 	local_lock_release(this_cpu_ptr(lock))
 
+/* localtry_lock_t variants */
+
+#define __localtry_lock_init(lock)				\
+do {								\
+	__local_lock_init(&(lock)->llock);			\
+	WRITE_ONCE((lock)->acquired, 0);			\
+} while (0)
+
+#define __localtry_lock(lock)					\
+	do {							\
+		localtry_lock_t *lt;				\
+		preempt_disable();				\
+		lt = this_cpu_ptr(lock);			\
+		local_lock_acquire(&lt->llock);			\
+		WRITE_ONCE(lt->acquired, 1);			\
+	} while (0)
+
+#define __localtry_lock_irq(lock)				\
+	do {							\
+		localtry_lock_t *lt;				\
+		local_irq_disable();				\
+		lt = this_cpu_ptr(lock);			\
+		local_lock_acquire(&lt->llock);			\
+		WRITE_ONCE(lt->acquired, 1);			\
+	} while (0)
+
+#define __localtry_lock_irqsave(lock, flags)			\
+	do {							\
+		localtry_lock_t *lt;				\
+		local_irq_save(flags);				\
+		lt = this_cpu_ptr(lock);			\
+		local_lock_acquire(&lt->llock);			\
+		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;				\
+		bool _ret;					\
+								\
+		local_irq_save(flags);				\
+		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;				\
+			local_irq_restore(flags);		\
+		}						\
+		_ret;						\
+	})
+
+#define __localtry_unlock(lock)					\
+	do {							\
+		localtry_lock_t *lt;				\
+		lt = this_cpu_ptr(lock);			\
+		WRITE_ONCE(lt->acquired, 0);			\
+		local_lock_release(&lt->llock);			\
+		preempt_enable();				\
+	} while (0)
+
+#define __localtry_unlock_irq(lock)				\
+	do {							\
+		localtry_lock_t *lt;				\
+		lt = this_cpu_ptr(lock);			\
+		WRITE_ONCE(lt->acquired, 0);			\
+		local_lock_release(&lt->llock);			\
+		local_irq_enable();				\
+	} while (0)
+
+#define __localtry_unlock_irqrestore(lock, flags)		\
+	do {							\
+		localtry_lock_t *lt;				\
+		lt = this_cpu_ptr(lock);			\
+		WRITE_ONCE(lt->acquired, 0);			\
+		local_lock_release(&lt->llock);			\
+		local_irq_restore(flags);			\
+	} while (0)
+
 #else /* !CONFIG_PREEMPT_RT */
 
 /*
@@ -125,8 +237,10 @@ do {								\
  * critical section while staying preemptible.
  */
 typedef spinlock_t local_lock_t;
+typedef spinlock_t localtry_lock_t;
 
 #define INIT_LOCAL_LOCK(lockname) __LOCAL_SPIN_LOCK_UNLOCKED((lockname))
+#define INIT_LOCALTRY_LOCK(lockname) INIT_LOCAL_LOCK(lockname)
 
 #define __local_lock_init(l)					\
 	do {							\
@@ -169,4 +283,36 @@ do {								\
 	spin_unlock(this_cpu_ptr((lock)));			\
 } while (0)
 
+/* localtry_lock_t variants */
+
+#define __localtry_lock_init(lock)			__local_lock_init(lock)
+#define __localtry_lock(lock)				__local_lock(lock)
+#define __localtry_lock_irq(lock)			__local_lock(lock)
+#define __localtry_lock_irqsave(lock, flags)		__local_lock_irqsave(lock, flags)
+#define __localtry_unlock(lock)				__local_unlock(lock)
+#define __localtry_unlock_irq(lock)			__local_unlock(lock)
+#define __localtry_unlock_irqrestore(lock, flags)	__local_unlock_irqrestore(lock, flags)
+
+#define __localtry_trylock(lock)				\
+	({							\
+		int __locked;					\
+								\
+		if (in_nmi() | in_hardirq()) {			\
+			__locked = 0;				\
+		} else {					\
+			migrate_disable();			\
+			__locked = spin_trylock(this_cpu_ptr((lock)));	\
+			if (!__locked)				\
+				migrate_enable();		\
+		}						\
+		__locked;					\
+	})
+
+#define __localtry_trylock_irqsave(lock, flags)			\
+	({							\
+		typecheck(unsigned long, flags);		\
+		flags = 0;					\
+		__localtry_trylock(lock);			\
+	})
+
 #endif /* CONFIG_PREEMPT_RT */