[RFC PATCH v1 04/25] printk-rb: add writer interface

[John Ogness <john.ogness@linutronix.de>]

Add the writer functions prb_reserve() and prb_commit(). These make
use of processor-reentrant spin locks to limit the number of possible
interruption scenarios for the writers.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 include/linux/printk_ringbuffer.h |  17 ++++
 lib/printk_ringbuffer.c           | 172 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 189 insertions(+)

diff --git a/include/linux/printk_ringbuffer.h b/include/linux/printk_ringbuffer.h
index 0e6e8dd0d01e..1aec9d5666b1 100644
--- a/include/linux/printk_ringbuffer.h
+++ b/include/linux/printk_ringbuffer.h
@@ -24,6 +24,18 @@ struct printk_ringbuffer {
 	atomic_t ctx;
 };
 
+struct prb_entry {
+	unsigned int size;
+	u64 seq;
+	char data[0];
+};
+
+struct prb_handle {
+	struct printk_ringbuffer *rb;
+	unsigned int cpu;
+	struct prb_entry *entry;
+};
+
 #define DECLARE_STATIC_PRINTKRB_CPULOCK(name)				\
 static DEFINE_PER_CPU(unsigned long, _##name##_percpu_irqflags);	\
 static struct prb_cpulock name = {					\
@@ -45,6 +57,11 @@ static struct printk_ringbuffer name = {				\
 	.ctx = ATOMIC_INIT(0),						\
 }
 
+/* writer interface */
+char *prb_reserve(struct prb_handle *h, struct printk_ringbuffer *rb,
+		  unsigned int size);
+void prb_commit(struct prb_handle *h);
+
 /* 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);
diff --git a/lib/printk_ringbuffer.c b/lib/printk_ringbuffer.c
index 28958b0cf774..90c7f9a9f861 100644
--- a/lib/printk_ringbuffer.c
+++ b/lib/printk_ringbuffer.c
@@ -2,6 +2,14 @@
 #include <linux/smp.h>
 #include <linux/printk_ringbuffer.h>
 
+#define PRB_SIZE(rb) (1 << rb->size_bits)
+#define PRB_SIZE_BITMASK(rb) (PRB_SIZE(rb) - 1)
+#define PRB_INDEX(rb, lpos) (lpos & PRB_SIZE_BITMASK(rb))
+#define PRB_WRAPS(rb, lpos) (lpos >> rb->size_bits)
+#define PRB_WRAP_LPOS(rb, lpos, xtra) \
+	((PRB_WRAPS(rb, lpos) + xtra) << rb->size_bits)
+#define PRB_DATA_ALIGN sizeof(long)
+
 static bool __prb_trylock(struct prb_cpulock *cpu_lock,
 			  unsigned int *cpu_store)
 {
@@ -75,3 +83,167 @@ void prb_unlock(struct prb_cpulock *cpu_lock, unsigned int cpu_store)
 
 	put_cpu();
 }
+
+static struct prb_entry *to_entry(struct printk_ringbuffer *rb,
+				  unsigned long lpos)
+{
+	char *buffer = rb->buffer;
+	buffer += PRB_INDEX(rb, lpos);
+	return (struct prb_entry *)buffer;
+}
+
+static int calc_next(struct printk_ringbuffer *rb, unsigned long tail,
+		     unsigned long lpos, int size, unsigned long *calced_next)
+{
+	unsigned long next_lpos;
+	int ret = 0;
+again:
+	next_lpos = lpos + size;
+	if (next_lpos - tail > PRB_SIZE(rb))
+		return -1;
+
+	if (PRB_WRAPS(rb, lpos) != PRB_WRAPS(rb, next_lpos)) {
+		lpos = PRB_WRAP_LPOS(rb, next_lpos, 0);
+		ret |= 1;
+		goto again;
+	}
+
+	*calced_next = next_lpos;
+	return ret;
+}
+
+static bool push_tail(struct printk_ringbuffer *rb, unsigned long tail)
+{
+	unsigned long new_tail;
+	struct prb_entry *e;
+	unsigned long head;
+
+	if (tail != atomic_long_read(&rb->tail))
+		return true;
+
+	e = to_entry(rb, tail);
+	if (e->size != -1)
+		new_tail = tail + e->size;
+	else
+		new_tail = PRB_WRAP_LPOS(rb, tail, 1);
+
+	/* make sure the new tail does not overtake the head */
+	head = atomic_long_read(&rb->head);
+	if (head - new_tail > PRB_SIZE(rb))
+		return false;
+
+	atomic_long_cmpxchg(&rb->tail, tail, new_tail);
+	return true;
+}
+
+/*
+ * prb_commit: Commit a reserved entry to the ring buffer.
+ * @h: An entry handle referencing the data entry to commit.
+ *
+ * Commit data that has been reserved using prb_reserve(). Once the data
+ * block has been committed, it can be invalidated at any time. If a writer
+ * is interested in using the data after committing, the writer should make
+ * its own copy first or use the prb_iter_ reader functions to access the
+ * data in the ring buffer.
+ *
+ * It is safe to call this function from any context and state.
+ */
+void prb_commit(struct prb_handle *h)
+{
+	struct printk_ringbuffer *rb = h->rb;
+	struct prb_entry *e;
+	unsigned long head;
+	unsigned long res;
+
+	for (;;) {
+		if (atomic_read(&rb->ctx) != 1) {
+			/* the interrupted context will fixup head */
+			atomic_dec(&rb->ctx);
+			break;
+		}
+		/* assign sequence numbers before moving head */
+		head = atomic_long_read(&rb->head);
+		res = atomic_long_read(&rb->reserve);
+		while (head != res) {
+			e = to_entry(rb, head);
+			if (e->size == -1) {
+				head = PRB_WRAP_LPOS(rb, head, 1);
+				continue;
+			}
+			e->seq = ++rb->seq;
+			head += e->size;
+		}
+		atomic_long_set_release(&rb->head, res);
+		atomic_dec(&rb->ctx);
+
+		if (atomic_long_read(&rb->reserve) == res)
+			break;
+		atomic_inc(&rb->ctx);
+	}
+
+	prb_unlock(rb->cpulock, h->cpu);
+}
+
+/*
+ * prb_reserve: Reserve an entry within a ring buffer.
+ * @h: An entry handle to be setup and reference an entry.
+ * @rb: A ring buffer to reserve data within.
+ * @size: The number of bytes to reserve.
+ *
+ * Reserve an entry of at least @size bytes to be used by the caller. If
+ * successful, the data region of the entry belongs to the caller and cannot
+ * be invalidated by any other task/context. For this reason, the caller
+ * should call prb_commit() as quickly as possible in order to avoid preventing
+ * other tasks/contexts from reserving data in the case that the ring buffer
+ * has wrapped.
+ *
+ * It is safe to call this function from any context and state.
+ *
+ * Returns a pointer to the reserved entry (and @h is setup to reference that
+ * entry) or NULL if it was not possible to reserve data.
+ */
+char *prb_reserve(struct prb_handle *h, struct printk_ringbuffer *rb,
+		  unsigned int size)
+{
+	unsigned long tail, res1, res2;
+	int ret;
+
+	if (size == 0)
+		return NULL;
+	size += sizeof(struct prb_entry);
+	size += PRB_DATA_ALIGN - 1;
+	size &= ~(PRB_DATA_ALIGN - 1);
+	if (size >= PRB_SIZE(rb))
+		return NULL;
+
+	h->rb = rb;
+	prb_lock(rb->cpulock, &h->cpu);
+
+	atomic_inc(&rb->ctx);
+
+	do {
+		for (;;) {
+			tail = atomic_long_read(&rb->tail);
+			res1 = atomic_long_read(&rb->reserve);
+			ret = calc_next(rb, tail, res1, size, &res2);
+			if (ret >= 0)
+				break;
+			if (!push_tail(rb, tail)) {
+				prb_commit(h);
+				return NULL;
+			}
+		}
+	} while (!atomic_long_try_cmpxchg_acquire(&rb->reserve, &res1, res2));
+
+	h->entry = to_entry(rb, res1);
+
+	if (ret) {
+		/* handle wrap */
+		h->entry->size = -1;
+		h->entry = to_entry(rb, PRB_WRAP_LPOS(rb, res2, 0));
+	}
+
+	h->entry->size = size;
+
+	return &h->entry->data[0];
+}
-- 
2.11.0