spsc_queue.h

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/*
 * Copyright 2017 Advanced Micro Devices, Inc.
 *
 * Permission is hereby granted, free of charge, to any person obtaining a
 * copy of this software and associated documentation files (the "Software"),
 * to deal in the Software without restriction, including without limitation
 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
 * and/or sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
 * THE COPYRIGHT HOLDER(S) OR AUTHOR(S) BE LIABLE FOR ANY CLAIM, DAMAGES OR
 * OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
 * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
 * OTHER DEALINGS IN THE SOFTWARE.
 *
 */
 
#ifndef DRM_SCHEDULER_SPSC_QUEUE_H_
#define DRM_SCHEDULER_SPSC_QUEUE_H_
 
#include <linux/atomic.h>
#include <linux/preempt.h>
 
struct spsc_node {
 
    /* Stores spsc_node* */
    struct spsc_node *next;
};
 
struct spsc_queue {
 
     struct spsc_node *head;
 
    /* atomic pointer to struct spsc_node* */
    atomic_long_t tail;
 
    atomic_t job_count;
};
 
static inline void spsc_queue_init(struct spsc_queue *queue)
{
    queue->head = NULL;
    atomic_long_set(&queue->tail, (long)&queue->head);
    atomic_set(&queue->job_count, 0);
}
 
static inline struct spsc_node *spsc_queue_peek(struct spsc_queue *queue)
{
    return queue->head;
}
 
static inline int spsc_queue_count(struct spsc_queue *queue)
{
    return atomic_read(&queue->job_count);
}
 
static inline bool spsc_queue_push(struct spsc_queue *queue, struct spsc_node *node)
{
    struct spsc_node **tail;
 
    node->next = NULL;
 
    preempt_disable();
 
    atomic_inc(&queue->job_count);
    smp_mb__after_atomic();
 
    tail = (struct spsc_node **)atomic_long_xchg(&queue->tail, (long)&node->next);
    WRITE_ONCE(*tail, node);
 
    /*
     * In case of first element verify new node will be visible to the consumer
     * thread when we ping the kernel thread that there is new work to do.
     */
    smp_wmb();
 
    preempt_enable();
 
    return tail == &queue->head;
}
 
 
static inline struct spsc_node *spsc_queue_pop(struct spsc_queue *queue)
{
    struct spsc_node *next, *node;
 
    /* Verify reading from memory and not the cache */
    smp_rmb();
 
    node = READ_ONCE(queue->head);
 
    if (!node)
        return NULL;
 
    next = READ_ONCE(node->next);
    WRITE_ONCE(queue->head, next);
 
    if (unlikely(!next)) {
        /* slowpath for the last element in the queue */
 
        if (atomic_long_cmpxchg(&queue->tail,
                (long)&node->next, (long) &queue->head) != (long)&node->next) {
            /* Updating tail failed wait for new next to appear */
            do {
                smp_rmb();
            } while (unlikely(!(queue->head = READ_ONCE(node->next))));
        }
    }
 
    atomic_dec(&queue->job_count);
    return node;
}
 
 
 
#endif /* DRM_SCHEDULER_SPSC_QUEUE_H_ */