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Subject[RFC PATCH v1 06/25] printk-rb: add blocking reader support
Date
Add a blocking read function for readers. An irq_work function is
used to signal the wait queue so that write notification can
be triggered from any context.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
include/linux/printk_ringbuffer.h | 20 ++++++++++++++++
lib/printk_ringbuffer.c | 49 +++++++++++++++++++++++++++++++++++++++
2 files changed, 69 insertions(+)

diff --git a/include/linux/printk_ringbuffer.h b/include/linux/printk_ringbuffer.h
index 5fdaf632c111..106f20ef8b4d 100644
--- a/include/linux/printk_ringbuffer.h
+++ b/include/linux/printk_ringbuffer.h
@@ -2,8 +2,10 @@
#ifndef _LINUX_PRINTK_RINGBUFFER_H
#define _LINUX_PRINTK_RINGBUFFER_H

+#include <linux/irq_work.h>
#include <linux/atomic.h>
#include <linux/percpu.h>
+#include <linux/wait.h>

struct prb_cpulock {
atomic_t owner;
@@ -22,6 +24,10 @@ struct printk_ringbuffer {

struct prb_cpulock *cpulock;
atomic_t ctx;
+
+ struct wait_queue_head *wq;
+ atomic_long_t wq_counter;
+ struct irq_work *wq_work;
};

struct prb_entry {
@@ -59,6 +65,15 @@ struct prb_iterator {
#define DECLARE_STATIC_PRINTKRB(name, szbits, cpulockptr) \
static char _##name##_buffer[1 << (szbits)] \
__aligned(__alignof__(long)); \
+static DECLARE_WAIT_QUEUE_HEAD(_##name##_wait); \
+static void _##name##_wake_work_func(struct irq_work *irq_work) \
+{ \
+ wake_up_interruptible_all(&_##name##_wait); \
+} \
+static struct irq_work _##name##_wake_work = { \
+ .func = _##name##_wake_work_func, \
+ .flags = IRQ_WORK_LAZY, \
+}; \
static struct printk_ringbuffer name = { \
.buffer = &_##name##_buffer[0], \
.size_bits = szbits, \
@@ -68,6 +83,9 @@ static struct printk_ringbuffer name = { \
.reserve = ATOMIC_LONG_INIT(-111 * sizeof(long)), \
.cpulock = cpulockptr, \
.ctx = ATOMIC_INIT(0), \
+ .wq = &_##name##_wait, \
+ .wq_counter = ATOMIC_LONG_INIT(0), \
+ .wq_work = &_##name##_wake_work, \
}

/* writer interface */
@@ -80,6 +98,8 @@ void prb_iter_init(struct prb_iterator *iter, struct printk_ringbuffer *rb,
u64 *seq);
void prb_iter_copy(struct prb_iterator *dest, struct prb_iterator *src);
int prb_iter_next(struct prb_iterator *iter, char *buf, int size, u64 *seq);
+int prb_iter_wait_next(struct prb_iterator *iter, char *buf, int size,
+ u64 *seq);
int prb_iter_data(struct prb_iterator *iter, char *buf, int size, u64 *seq);

/* utility functions */
diff --git a/lib/printk_ringbuffer.c b/lib/printk_ringbuffer.c
index 1d1e886a0966..c2ddf4cb9f92 100644
--- a/lib/printk_ringbuffer.c
+++ b/lib/printk_ringbuffer.c
@@ -1,4 +1,5 @@
// SPDX-License-Identifier: GPL-2.0
+#include <linux/sched.h>
#include <linux/smp.h>
#include <linux/string.h>
#include <linux/errno.h>
@@ -154,6 +155,7 @@ static bool push_tail(struct printk_ringbuffer *rb, unsigned long tail)
void prb_commit(struct prb_handle *h)
{
struct printk_ringbuffer *rb = h->rb;
+ bool changed = false;
struct prb_entry *e;
unsigned long head;
unsigned long res;
@@ -175,6 +177,7 @@ void prb_commit(struct prb_handle *h)
}
e->seq = ++rb->seq;
head += e->size;
+ changed = true;
}
atomic_long_set_release(&rb->head, res);
atomic_dec(&rb->ctx);
@@ -185,6 +188,12 @@ void prb_commit(struct prb_handle *h)
}

prb_unlock(rb->cpulock, h->cpu);
+
+ if (changed) {
+ atomic_long_inc(&rb->wq_counter);
+ if (wq_has_sleeper(rb->wq))
+ irq_work_queue(rb->wq_work);
+ }
}

/*
@@ -437,3 +446,43 @@ int prb_iter_next(struct prb_iterator *iter, char *buf, int size, u64 *seq)

return 1;
}
+
+/*
+ * prb_iter_wait_next: Advance to the next record, blocking if none available.
+ * @iter: Iterator tracking the current position.
+ * @buf: A buffer to store the data of the next record. May be NULL.
+ * @size: The size of @buf. (Ignored if @buf is NULL.)
+ * @seq: The sequence number of the next record. May be NULL.
+ *
+ * If a next record is already available, this function works like
+ * prb_iter_next(). Otherwise block interruptible until a next record is
+ * available.
+ *
+ * When a next record is available, @iter is advanced and (if specified)
+ * the data and/or sequence number of that record are provided.
+ *
+ * This function might sleep.
+ *
+ * Returns 1 if @iter was advanced, -EINVAL if @iter is now invalid, or
+ * -ERESTARTSYS if interrupted by a signal.
+ */
+int prb_iter_wait_next(struct prb_iterator *iter, char *buf, int size, u64 *seq)
+{
+ unsigned long last_seen;
+ int ret;
+
+ for (;;) {
+ last_seen = atomic_long_read(&iter->rb->wq_counter);
+
+ ret = prb_iter_next(iter, buf, size, seq);
+ if (ret != 0)
+ break;
+
+ ret = wait_event_interruptible(*iter->rb->wq,
+ last_seen != atomic_long_read(&iter->rb->wq_counter));
+ if (ret < 0)
+ break;
+ }
+
+ return ret;
+}
--
2.11.0
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