locking: Introduce local_lock()

preempt_disable() and local_irq_disable/save() are in principle per CPU big
kernel locks. This has several downsides:

  - The protection scope is unknown

  - Violation of protection rules is hard to detect by instrumentation

  - For PREEMPT_RT such sections, unless in low level critical code, can
    violate the preemptability constraints.

To address this PREEMPT_RT introduced the concept of local_locks which are
strictly per CPU.

The lock operations map to preempt_disable(), local_irq_disable/save() and
the enabling counterparts on non RT enabled kernels.

If lockdep is enabled local locks gain a lock map which tracks the usage
context. This will catch cases where an area is protected by
preempt_disable() but the access also happens from interrupt context. local
locks have identified quite a few such issues over the years, the most
recent example is:

  b7d5dc21072cd ("random: add a spinlock_t to struct batched_entropy")

Aside of the lockdep coverage this also improves code readability as it
precisely annotates the protection scope.

PREEMPT_RT substitutes these local locks with 'sleeping' spinlocks to
protect such sections while maintaining preemtability and CPU locality.

local locks can replace:

  - preempt_enable()/disable() pairs
  - local_irq_disable/enable() pairs
  - local_irq_save/restore() pairs

They are also used to replace code which implicitly disables preemption
like:

  - get_cpu()/put_cpu()
  - get_cpu_var()/put_cpu_var()

with PREEMPT_RT friendly constructs.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20200527201119.1692513-2-bigeasy@linutronix.de

1 files changed, 54 insertions, 0 deletions

diff --git a/include/linux/local_lock.h b/include/linux/local_lock.h
new file mode 100644
index 000000000000..e55010fa7329
--- /dev/null
+++ b/include/linux/local_lock.h
@@ -0,0 +1,54 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_LOCAL_LOCK_H
+#define _LINUX_LOCAL_LOCK_H
+
+#include <linux/local_lock_internal.h>
+
+/**
+ * local_lock_init - Runtime initialize a lock instance
+ */
+#define local_lock_init(lock)		__local_lock_init(lock)
+
+/**
+ * local_lock - Acquire a per CPU local lock
+ * @lock:	The lock variable
+ */
+#define local_lock(lock)		__local_lock(lock)
+
+/**
+ * local_lock_irq - Acquire a per CPU local lock and disable interrupts
+ * @lock:	The lock variable
+ */
+#define local_lock_irq(lock)		__local_lock_irq(lock)
+
+/**
+ * local_lock_irqsave - Acquire a per CPU local lock, save and disable
+ *			 interrupts
+ * @lock:	The lock variable
+ * @flags:	Storage for interrupt flags
+ */
+#define local_lock_irqsave(lock, flags)				\
+	__local_lock_irqsave(lock, flags)
+
+/**
+ * local_unlock - Release a per CPU local lock
+ * @lock:	The lock variable
+ */
+#define local_unlock(lock)		__local_unlock(lock)
+
+/**
+ * local_unlock_irq - Release a per CPU local lock and enable interrupts
+ * @lock:	The lock variable
+ */
+#define local_unlock_irq(lock)		__local_unlock_irq(lock)
+
+/**
+ * local_unlock_irqrestore - Release a per CPU local lock and restore
+ *			      interrupt flags
+ * @lock:	The lock variable
+ * @flags:      Interrupt flags to restore
+ */
+#define local_unlock_irqrestore(lock, flags)			\
+	__local_unlock_irqrestore(lock, flags)
+
+#endif