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SubjectRe: [RFC, 2.6] a simple FIFO implementation
On Fri, Sep 17, 2004 at 12:44:35PM +0100, Hugh Dickins wrote:

> On Fri, 17 Sep 2004, Stelian Pop wrote:
> > - if the fifo becomes empty after a get() sets in = out = 0
> > so only a memcpy() will be needed not two in the next put/get.
>
> Within the lockless __kfifo_get? Doesn't that violate an essential
> property of such a circular buffer, that the producer manipulates
> only the "in" index and the consumer only the "out" index?
> Within the locking version's kfifo_get wrapper, perhaps.

Indeed. Moved the optimization to the wrapper and added some
documentation to __kfifo_get()/__kfifo_put() saying that you don't
need extra locking if you have only one concurrent reader and one
concurrent writer.

Thanks.

Stelian.

Signed-off-by: Stelian Pop <stelian@popies.net>

--- /dev/null 2004-02-23 22:02:56.000000000 +0100
+++ linux-2.6/include/linux/kfifo.h 2004-09-17 14:18:16.828691528 +0200
@@ -0,0 +1,138 @@
+/*
+ * A simple kernel FIFO implementation.
+ *
+ * Copyright (C) 2004 Stelian Pop <stelian@popies.net>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ *
+ */
+#ifndef _LINUX_KFIFO_H
+#define _LINUX_KFIFO_H
+
+#ifdef __KERNEL__
+
+#include <linux/kernel.h>
+#include <linux/spinlock.h>
+
+struct kfifo {
+ unsigned char *buffer; /* the buffer holding the data */
+ unsigned int size; /* the size of the allocated buffer */
+ unsigned int in; /* data is added at offset (in % size) */
+ unsigned int out; /* data is extracted from off. (out % size) */
+ spinlock_t lock; /* protects concurrent modifications */
+};
+
+extern struct kfifo *kfifo_alloc(unsigned int size, int gfp_mask);
+extern void kfifo_free(struct kfifo *fifo);
+extern void __kfifo_reset(struct kfifo *fifo);
+extern unsigned int __kfifo_put(struct kfifo *fifo,
+ unsigned char *buffer, unsigned int len);
+extern unsigned int __kfifo_get(struct kfifo *fifo,
+ unsigned char *buffer, unsigned int len);
+extern unsigned int __kfifo_len(struct kfifo *fifo);
+
+/*
+ * kfifo_reset - removes the entire FIFO contents
+ * @fifo: the fifo to be emptied.
+ */
+static inline void kfifo_reset(struct kfifo *fifo)
+{
+ unsigned long flags;
+
+ spin_lock_irqsave(&fifo->lock, flags);
+
+ __kfifo_reset(fifo);
+
+ spin_unlock_irqrestore(&fifo->lock, flags);
+}
+
+/*
+ * kfifo_put - puts some data into the FIFO
+ * @fifo: the fifo to be used.
+ * @buffer: the data to be added.
+ * @len: the length of the data to be added.
+ *
+ * This function copies at most 'len' bytes from the 'buffer' into
+ * the FIFO depending on the free space, and returns the number of
+ * bytes copied.
+ */
+static inline unsigned int kfifo_put(struct kfifo *fifo,
+ unsigned char *buffer, unsigned int len)
+{
+ unsigned long flags;
+ unsigned int ret;
+
+ spin_lock_irqsave(&fifo->lock, flags);
+
+ ret = __kfifo_put(fifo, buffer, len);
+
+ spin_unlock_irqrestore(&fifo->lock, flags);
+
+ return ret;
+}
+
+/*
+ * kfifo_get - gets some data from the FIFO
+ * @fifo: the fifo to be used.
+ * @buffer: where the data must be copied.
+ * @len: the size of the destination buffer.
+ *
+ * This function copies at most 'len' bytes from the FIFO into the
+ * 'buffer' and returns the number of copied bytes.
+ */
+static inline unsigned int kfifo_get(struct kfifo *fifo,
+ unsigned char *buffer, unsigned int len)
+{
+ unsigned long flags;
+ unsigned int ret;
+
+ spin_lock_irqsave(&fifo->lock, flags);
+
+ ret = __kfifo_get(fifo, buffer, len);
+
+ /*
+ * optimization: if the FIFO is empty, set the indices to 0
+ * so we don't wrap the next time
+ */
+ if (fifo->in == fifo->out)
+ fifo->in = fifo->out = 0;
+
+ spin_unlock_irqrestore(&fifo->lock, flags);
+
+ return ret;
+}
+
+/*
+ * kfifo_len - returns the number of bytes available in the FIFO
+ * @fifo: the fifo to be used.
+ */
+static inline unsigned int kfifo_len(struct kfifo *fifo)
+{
+ unsigned long flags;
+ unsigned int ret;
+
+ spin_lock_irqsave(&fifo->lock, flags);
+
+ ret = __kfifo_len(fifo);
+
+ spin_unlock_irqrestore(&fifo->lock, flags);
+
+ return ret;
+}
+
+#else
+#warning "don't include kernel headers in userspace"
+#endif /* __KERNEL__ */
+#endif
--- /dev/null 2004-02-23 22:02:56.000000000 +0100
+++ linux-2.6/kernel/kfifo.c 2004-09-17 14:20:36.355480232 +0200
@@ -0,0 +1,148 @@
+/*
+ * A simple kernel FIFO implementation.
+ *
+ * Copyright (C) 2004 Stelian Pop <stelian@popies.net>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ *
+ */
+
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/slab.h>
+#include <linux/kfifo.h>
+
+/*
+ * kfifo_alloc - allocates a new FIFO
+ * @size: the size of the internal buffer.
+ * @gfp_mask: get_free_pages mask, passed to kmalloc()
+ */
+struct kfifo *kfifo_alloc(unsigned int size, int gfp_mask)
+{
+ struct kfifo *fifo;
+
+ fifo = kmalloc(sizeof(struct kfifo), gfp_mask);
+ if (!fifo)
+ return NULL;
+
+ fifo->buffer = kmalloc(size, gfp_mask);
+ if (!fifo->buffer) {
+ kfree(fifo);
+ return NULL;
+ }
+
+ fifo->in = fifo->out = 0;
+ fifo->size = size;
+ fifo->lock = SPIN_LOCK_UNLOCKED;
+
+ return fifo;
+}
+EXPORT_SYMBOL(kfifo_alloc);
+
+/*
+ * kfifo_free - frees the FIFO
+ * @fifo: the fifo to be freed.
+ */
+void kfifo_free(struct kfifo *fifo)
+{
+ kfree(fifo->buffer);
+ kfree(fifo);
+}
+EXPORT_SYMBOL(kfifo_free);
+
+/*
+ * __kfifo_reset - removes the entire FIFO contents, no locking version
+ * @fifo: the fifo to be emptied.
+ */
+void __kfifo_reset(struct kfifo *fifo)
+{
+ fifo->in = fifo->out = 0;
+}
+EXPORT_SYMBOL(__kfifo_reset);
+
+/*
+ * __kfifo_put - puts some data into the FIFO, no locking version
+ * @fifo: the fifo to be used.
+ * @buffer: the data to be added.
+ * @len: the length of the data to be added.
+ *
+ * This function copies at most 'len' bytes from the 'buffer' into
+ * the FIFO depending on the free space, and returns the number of
+ * bytes copied.
+ *
+ * Note that with only one concurrent reader and one concurrent
+ * writer, you don't need extra locking to use these functions.
+ */
+unsigned int __kfifo_put(struct kfifo *fifo,
+ unsigned char *buffer, unsigned int len)
+{
+ unsigned int l;
+
+ len = min(len, fifo->size - fifo->in + fifo->out);
+
+ /* first put the data starting from fifo->in to buffer end */
+ l = min(len, fifo->size - (fifo->in % fifo->size));
+ memcpy(fifo->buffer + (fifo->in % fifo->size), buffer, l);
+
+ /* then put the rest (if any) at the beginning of the buffer */
+ memcpy(fifo->buffer, buffer + l, len - l);
+
+ fifo->in += len;
+
+ return len;
+}
+EXPORT_SYMBOL(__kfifo_put);
+
+/*
+ * kfifo_get - gets some data from the FIFO, no locking version
+ * @fifo: the fifo to be used.
+ * @buffer: where the data must be copied.
+ * @len: the size of the destination buffer.
+ *
+ * This function copies at most 'len' bytes from the FIFO into the
+ * 'buffer' and returns the number of copied bytes.
+ *
+ * Note that with only one concurrent reader and one concurrent
+ * writer, you don't need extra locking to use these functions.
+ */
+unsigned int __kfifo_get(struct kfifo *fifo,
+ unsigned char *buffer, unsigned int len)
+{
+ unsigned int l;
+
+ len = min(len, fifo->in - fifo->out);
+
+ /* first get the data from fifo->out until the end of the buffer */
+ l = min(len, fifo->size - (fifo->out % fifo->size));
+ memcpy(buffer, fifo->buffer + (fifo->out % fifo->size), l);
+
+ /* then get the rest (if any) from the beginning of the buffer */
+ memcpy(buffer, fifo->buffer + l, len - l);
+
+ fifo->out += len;
+
+ return len;
+}
+EXPORT_SYMBOL(__kfifo_get);
+
+/*
+ * kfifo_len - returns the number of bytes available in the FIFO, no locking version
+ * @fifo: the fifo to be used.
+ */
+unsigned int __kfifo_len(struct kfifo *fifo)
+{
+ return fifo->in - fifo->out;
+}
+EXPORT_SYMBOL(__kfifo_len);
--- linux-2.6/kernel/Makefile.orig 2004-09-16 12:27:29.000000000 +0200
+++ linux-2.6/kernel/Makefile 2004-09-16 11:58:26.000000000 +0200
@@ -7,7 +7,7 @@
sysctl.o capability.o ptrace.o timer.o user.o \
signal.o sys.o kmod.o workqueue.o pid.o \
rcupdate.o intermodule.o extable.o params.o posix-timers.o \
- kthread.o
+ kthread.o kfifo.o

obj-$(CONFIG_FUTEX) += futex.o
obj-$(CONFIG_GENERIC_ISA_DMA) += dma.o
--
Stelian Pop <stelian@popies.net>
-
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