Linux开发--使用Memory barrier实现无锁环形缓冲区

一 说明

    涉及到并发编程中较底层的memory barrier相关知识，本人水平有限，在此不展开讲述，读者自行查阅。

二 代码

    Linux内核中，实现了一个无锁（只有一个读线程和一个写线程时）环形缓冲区 kfifo 使用到了 Memory barrier，实现源码如下：

/* * A simple kernel FIFO implementation. * * Copyright (C) 2004 Stelian Pop <[e-mail]> * * 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/err.h>#include <linux/kfifo.h>#include <linux/log2.h> /** * kfifo_init - allocates a new FIFO using a preallocated buffer * @buffer: the preallocated buffer to be used. * @size: the size of the internal buffer, this have to be a power of 2. * @gfp_mask: get_free_pages mask, passed to kmalloc() * @lock: the lock to be used to protect the fifo buffer * * Do NOT pass the kfifo to kfifo_free() after use! Simply free the * &struct kfifo with kfree(). */struct kfifo *kfifo_init(unsigned char *buffer, unsigned int size,                         gfp_t gfp_mask, spinlock_t *lock){    struct kfifo *fifo;     /* size must be a power of 2 */    BUG_ON(!is_power_of_2(size));     fifo = kmalloc(sizeof(struct kfifo), gfp_mask);    if (!fifo)        return ERR_PTR(-ENOMEM);     fifo->buffer = buffer;    fifo->size = size;    fifo->in = fifo->out = 0;    fifo->lock = lock;     return fifo;}EXPORT_SYMBOL(kfifo_init); /** * kfifo_alloc - allocates a new FIFO and its internal buffer * @size: the size of the internal buffer to be allocated. * @gfp_mask: get_free_pages mask, passed to kmalloc() * @lock: the lock to be used to protect the fifo buffer * * The size will be rounded-up to a power of 2. */struct kfifo *kfifo_alloc(unsigned int size, gfp_t gfp_mask, spinlock_t *lock){    unsigned char *buffer;    struct kfifo *ret;     /*     * round up to the next power of 2, since our 'let the indices     * wrap' technique works only in this case.     */    if (!is_power_of_2(size)) {        BUG_ON(size > 0x80000000);        size = roundup_pow_of_two(size);    }     buffer = kmalloc(size, gfp_mask);    if (!buffer)        return ERR_PTR(-ENOMEM);     ret = kfifo_init(buffer, size, gfp_mask, lock);     if (IS_ERR(ret))        kfree(buffer);     return ret;}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_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,                         const unsigned char *buffer, unsigned int len){    unsigned int l;     len = min(len, fifo->size - fifo->in + fifo->out);     /*     * Ensure that we sample the fifo->out index -before- we     * start putting bytes into the kfifo.     */     smp_mb();     /* first put the data starting from fifo->in to buffer end */    l = min(len, fifo->size - (fifo->in & (fifo->size - 1)));    memcpy(fifo->buffer + (fifo->in & (fifo->size - 1)), buffer, l);     /* then put the rest (if any) at the beginning of the buffer */    memcpy(fifo->buffer, buffer + l, len - l);     /*     * Ensure that we add the bytes to the kfifo -before-     * we update the fifo->in index.     */     smp_wmb();     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);     /*     * Ensure that we sample the fifo->in index -before- we     * start removing bytes from the kfifo.     */     smp_rmb();     /* first get the data from fifo->out until the end of the buffer */    l = min(len, fifo->size - (fifo->out & (fifo->size - 1)));    memcpy(buffer, fifo->buffer + (fifo->out & (fifo->size - 1)), l);     /* then get the rest (if any) from the beginning of the buffer */    memcpy(buffer + l, fifo->buffer, len - l);     /*     * Ensure that we remove the bytes from the kfifo -before-     * we update the fifo->out index.     */     smp_mb();     fifo->out += len;     return len;}EXPORT_SYMBOL(__kfifo_get);

上述代码在实现环形缓冲区时使用的一些技巧：

  1 使用与操作来求取环形缓冲区的下标，相比取余操作来求取下标的做法效率要高不少。使用与操作求取下标的前提是环形缓冲区的大小必须是 2 的 N 次方，换而言之就是说环形缓冲区的大小为一个仅有一个 1 的二进制数，那么 index & (size – 1) 则为求取的下标（这不难理解）
  2 使用了 in 和 out 两个索引且 in 和 out 是一直递增的（此做法比较巧妙），这样能够避免一些复杂的条件判断（某些实现下，in == out 时还无法区分缓冲区是空还是满）