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