printk-rb: add prb locking functions

Add processor-reentrant spin locking functions. These allow
restricting the number of possible contexts to 2, which can simplify
implementing code that also supports NMI interruptions.

    prb_lock();

    /*
     * This code is synchronized with all contexts
     * except an NMI on the same processor.
     */

    prb_unlock();

In order to support printk's emergency messages, a
processor-reentrant spin lock will be used to control raw access to
the emergency console. However, it must be the same
processor-reentrant spin lock as the one used by the ring buffer,
otherwise a deadlock can occur:

    CPU1: printk lock -> emergency -> serial lock
    CPU2: serial lock -> printk lock

By making the processor-reentrant implemtation available externally,
printk can use the same atomic_t for the ring buffer as for the
emergency console and thus avoid the above deadlock.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

include/linux/printk_ringbuffer.h

@@ -0,0 +1,24 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_PRINTK_RINGBUFFER_H
+#define _LINUX_PRINTK_RINGBUFFER_H
+
+#include <linux/atomic.h>
+#include <linux/percpu.h>
+
+struct prb_cpulock {
+	atomic_t owner;
+	unsigned long __percpu *irqflags;
+};
+
+#define DECLARE_STATIC_PRINTKRB_CPULOCK(name)				\
+static DEFINE_PER_CPU(unsigned long, _##name##_percpu_irqflags);	\
+static struct prb_cpulock name = {					\
+	.owner = ATOMIC_INIT(-1),					\
+	.irqflags = &_##name##_percpu_irqflags,				\
+}
+
+/* utility functions */
+void prb_lock(struct prb_cpulock *cpu_lock, unsigned int *cpu_store);
+void prb_unlock(struct prb_cpulock *cpu_lock, unsigned int cpu_store);
+
+#endif /*_LINUX_PRINTK_RINGBUFFER_H */

lib/Makefile

@@ -26,7 +26,7 @@ endif
 
 lib-y := ctype.o string.o vsprintf.o cmdline.o \
 	 rbtree.o radix-tree.o timerqueue.o xarray.o \
-	 idr.o extable.o \
+	 idr.o extable.o printk_ringbuffer.o \
 	 sha1.o chacha.o irq_regs.o argv_split.o \
 	 flex_proportions.o ratelimit.o show_mem.o \
 	 is_single_threaded.o plist.o decompress.o kobject_uevent.o \

lib/printk_ringbuffer.c

@@ -0,0 +1,77 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/smp.h>
+#include <linux/printk_ringbuffer.h>
+
+static bool __prb_trylock(struct prb_cpulock *cpu_lock,
+			  unsigned int *cpu_store)
+{
+	unsigned long *flags;
+	unsigned int cpu;
+
+	cpu = get_cpu();
+
+	*cpu_store = atomic_read(&cpu_lock->owner);
+	/* memory barrier to ensure the current lock owner is visible */
+	smp_rmb();
+	if (*cpu_store == -1) {
+		flags = per_cpu_ptr(cpu_lock->irqflags, cpu);
+		local_irq_save(*flags);
+		if (atomic_try_cmpxchg_acquire(&cpu_lock->owner,
+					       cpu_store, cpu)) {
+			return true;
+		}
+		local_irq_restore(*flags);
+	} else if (*cpu_store == cpu) {
+		return true;
+	}
+
+	put_cpu();
+	return false;
+}
+
+/*
+ * prb_lock: Perform a processor-reentrant spin lock.
+ * @cpu_lock: A pointer to the lock object.
+ * @cpu_store: A "flags" pointer to store lock status information.
+ *
+ * If no processor has the lock, the calling processor takes the lock and
+ * becomes the owner. If the calling processor is already the owner of the
+ * lock, this function succeeds immediately. If lock is locked by another
+ * processor, this function spins until the calling processor becomes the
+ * owner.
+ *
+ * It is safe to call this function from any context and state.
+ */
+void prb_lock(struct prb_cpulock *cpu_lock, unsigned int *cpu_store)
+{
+	for (;;) {
+		if (__prb_trylock(cpu_lock, cpu_store))
+			break;
+		cpu_relax();
+	}
+}
+
+/*
+ * prb_unlock: Perform a processor-reentrant spin unlock.
+ * @cpu_lock: A pointer to the lock object.
+ * @cpu_store: A "flags" object storing lock status information.
+ *
+ * Release the lock. The calling processor must be the owner of the lock.
+ *
+ * It is safe to call this function from any context and state.
+ */
+void prb_unlock(struct prb_cpulock *cpu_lock, unsigned int cpu_store)
+{
+	unsigned long *flags;
+	unsigned int cpu;
+
+	cpu = atomic_read(&cpu_lock->owner);
+	atomic_set_release(&cpu_lock->owner, cpu_store);
+
+	if (cpu_store == -1) {
+		flags = per_cpu_ptr(cpu_lock->irqflags, cpu);
+		local_irq_restore(*flags);
+	}
+
+	put_cpu();
+}