The Design and Implementation of the FreeBSD Operating System, Second Edition
Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)


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FreeBSD/Linux Kernel Cross Reference
sys/sys/buf_ring.h

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    1 /*-
    2  * Copyright (c) 2007-2009 Kip Macy <kmacy@freebsd.org>
    3  * All rights reserved.
    4  *
    5  * Redistribution and use in source and binary forms, with or without
    6  * modification, are permitted provided that the following conditions
    7  * are met:
    8  * 1. Redistributions of source code must retain the above copyright
    9  *    notice, this list of conditions and the following disclaimer.
   10  * 2. Redistributions in binary form must reproduce the above copyright
   11  *    notice, this list of conditions and the following disclaimer in the
   12  *    documentation and/or other materials provided with the distribution.
   13  *
   14  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
   15  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   16  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   17  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
   18  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   19  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   20  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   21  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   22  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   23  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   24  * SUCH DAMAGE.
   25  *
   26  * $FreeBSD: releng/10.1/sys/sys/buf_ring.h 246482 2013-02-07 15:20:54Z rrs $
   27  *
   28  */
   29 
   30 #ifndef _SYS_BUF_RING_H_
   31 #define _SYS_BUF_RING_H_
   32 
   33 #include <machine/cpu.h>
   34 
   35 #if defined(INVARIANTS) && !defined(DEBUG_BUFRING)
   36 #define DEBUG_BUFRING 1
   37 #endif
   38 
   39 #ifdef DEBUG_BUFRING
   40 #include <sys/lock.h>
   41 #include <sys/mutex.h>
   42 #endif
   43 
   44 struct buf_ring {
   45         volatile uint32_t       br_prod_head;
   46         volatile uint32_t       br_prod_tail;   
   47         int                     br_prod_size;
   48         int                     br_prod_mask;
   49         uint64_t                br_drops;
   50         volatile uint32_t       br_cons_head __aligned(CACHE_LINE_SIZE);
   51         volatile uint32_t       br_cons_tail;
   52         int                     br_cons_size;
   53         int                     br_cons_mask;
   54 #ifdef DEBUG_BUFRING
   55         struct mtx              *br_lock;
   56 #endif  
   57         void                    *br_ring[0] __aligned(CACHE_LINE_SIZE);
   58 };
   59 
   60 /*
   61  * multi-producer safe lock-free ring buffer enqueue
   62  *
   63  */
   64 static __inline int
   65 buf_ring_enqueue(struct buf_ring *br, void *buf)
   66 {
   67         uint32_t prod_head, prod_next;
   68         uint32_t cons_tail;
   69 #ifdef DEBUG_BUFRING
   70         int i;
   71         for (i = br->br_cons_head; i != br->br_prod_head;
   72              i = ((i + 1) & br->br_cons_mask))
   73                 if(br->br_ring[i] == buf)
   74                         panic("buf=%p already enqueue at %d prod=%d cons=%d",
   75                             buf, i, br->br_prod_tail, br->br_cons_tail);
   76 #endif  
   77         critical_enter();
   78         do {
   79                 prod_head = br->br_prod_head;
   80                 cons_tail = br->br_cons_tail;
   81 
   82                 prod_next = (prod_head + 1) & br->br_prod_mask;
   83                 
   84                 if (prod_next == cons_tail) {
   85                         br->br_drops++;
   86                         critical_exit();
   87                         return (ENOBUFS);
   88                 }
   89         } while (!atomic_cmpset_int(&br->br_prod_head, prod_head, prod_next));
   90 #ifdef DEBUG_BUFRING
   91         if (br->br_ring[prod_head] != NULL)
   92                 panic("dangling value in enqueue");
   93 #endif  
   94         br->br_ring[prod_head] = buf;
   95 
   96         /*
   97          * The full memory barrier also avoids that br_prod_tail store
   98          * is reordered before the br_ring[prod_head] is full setup.
   99          */
  100         mb();
  101 
  102         /*
  103          * If there are other enqueues in progress
  104          * that preceeded us, we need to wait for them
  105          * to complete 
  106          */   
  107         while (br->br_prod_tail != prod_head)
  108                 cpu_spinwait();
  109         br->br_prod_tail = prod_next;
  110         critical_exit();
  111         return (0);
  112 }
  113 
  114 /*
  115  * multi-consumer safe dequeue 
  116  *
  117  */
  118 static __inline void *
  119 buf_ring_dequeue_mc(struct buf_ring *br)
  120 {
  121         uint32_t cons_head, cons_next;
  122         uint32_t prod_tail;
  123         void *buf;
  124         int success;
  125 
  126         critical_enter();
  127         do {
  128                 cons_head = br->br_cons_head;
  129                 prod_tail = br->br_prod_tail;
  130 
  131                 cons_next = (cons_head + 1) & br->br_cons_mask;
  132                 
  133                 if (cons_head == prod_tail) {
  134                         critical_exit();
  135                         return (NULL);
  136                 }
  137                 
  138                 success = atomic_cmpset_int(&br->br_cons_head, cons_head,
  139                     cons_next);
  140         } while (success == 0);         
  141 
  142         buf = br->br_ring[cons_head];
  143 #ifdef DEBUG_BUFRING
  144         br->br_ring[cons_head] = NULL;
  145 #endif
  146 
  147         /*
  148          * The full memory barrier also avoids that br_ring[cons_read]
  149          * load is reordered after br_cons_tail is set.
  150          */
  151         mb();
  152         
  153         /*
  154          * If there are other dequeues in progress
  155          * that preceeded us, we need to wait for them
  156          * to complete 
  157          */   
  158         while (br->br_cons_tail != cons_head)
  159                 cpu_spinwait();
  160 
  161         br->br_cons_tail = cons_next;
  162         critical_exit();
  163 
  164         return (buf);
  165 }
  166 
  167 /*
  168  * single-consumer dequeue 
  169  * use where dequeue is protected by a lock
  170  * e.g. a network driver's tx queue lock
  171  */
  172 static __inline void *
  173 buf_ring_dequeue_sc(struct buf_ring *br)
  174 {
  175         uint32_t cons_head, cons_next, cons_next_next;
  176         uint32_t prod_tail;
  177         void *buf;
  178         
  179         cons_head = br->br_cons_head;
  180         prod_tail = br->br_prod_tail;
  181         
  182         cons_next = (cons_head + 1) & br->br_cons_mask;
  183         cons_next_next = (cons_head + 2) & br->br_cons_mask;
  184         
  185         if (cons_head == prod_tail) 
  186                 return (NULL);
  187 
  188 #ifdef PREFETCH_DEFINED 
  189         if (cons_next != prod_tail) {           
  190                 prefetch(br->br_ring[cons_next]);
  191                 if (cons_next_next != prod_tail) 
  192                         prefetch(br->br_ring[cons_next_next]);
  193         }
  194 #endif
  195         br->br_cons_head = cons_next;
  196         buf = br->br_ring[cons_head];
  197 
  198 #ifdef DEBUG_BUFRING
  199         br->br_ring[cons_head] = NULL;
  200         if (!mtx_owned(br->br_lock))
  201                 panic("lock not held on single consumer dequeue");
  202         if (br->br_cons_tail != cons_head)
  203                 panic("inconsistent list cons_tail=%d cons_head=%d",
  204                     br->br_cons_tail, cons_head);
  205 #endif
  206         br->br_cons_tail = cons_next;
  207         return (buf);
  208 }
  209 
  210 /*
  211  * single-consumer advance after a peek
  212  * use where it is protected by a lock
  213  * e.g. a network driver's tx queue lock
  214  */
  215 static __inline void
  216 buf_ring_advance_sc(struct buf_ring *br)
  217 {
  218         uint32_t cons_head, cons_next;
  219         uint32_t prod_tail;
  220         
  221         cons_head = br->br_cons_head;
  222         prod_tail = br->br_prod_tail;
  223         
  224         cons_next = (cons_head + 1) & br->br_cons_mask;
  225         if (cons_head == prod_tail) 
  226                 return;
  227         br->br_cons_head = cons_next;
  228 #ifdef DEBUG_BUFRING
  229         br->br_ring[cons_head] = NULL;
  230 #endif
  231         br->br_cons_tail = cons_next;
  232 }
  233 
  234 /*
  235  * Used to return a buffer (most likely already there)
  236  * to the top od the ring. The caller should *not*
  237  * have used any dequeue to pull it out of the ring
  238  * but instead should have used the peek() function.
  239  * This is normally used where the transmit queue
  240  * of a driver is full, and an mubf must be returned.
  241  * Most likely whats in the ring-buffer is what
  242  * is being put back (since it was not removed), but
  243  * sometimes the lower transmit function may have
  244  * done a pullup or other function that will have
  245  * changed it. As an optimzation we always put it
  246  * back (since jhb says the store is probably cheaper),
  247  * if we have to do a multi-queue version we will need
  248  * the compare and an atomic.
  249  */
  250 static __inline void
  251 buf_ring_putback_sc(struct buf_ring *br, void *new)
  252 {
  253         KASSERT(br->br_cons_head != br->br_prod_tail, 
  254                 ("Buf-Ring has none in putback")) ;
  255         br->br_ring[br->br_cons_head] = new;
  256 }
  257 
  258 /*
  259  * return a pointer to the first entry in the ring
  260  * without modifying it, or NULL if the ring is empty
  261  * race-prone if not protected by a lock
  262  */
  263 static __inline void *
  264 buf_ring_peek(struct buf_ring *br)
  265 {
  266 
  267 #ifdef DEBUG_BUFRING
  268         if ((br->br_lock != NULL) && !mtx_owned(br->br_lock))
  269                 panic("lock not held on single consumer dequeue");
  270 #endif  
  271         /*
  272          * I believe it is safe to not have a memory barrier
  273          * here because we control cons and tail is worst case
  274          * a lagging indicator so we worst case we might
  275          * return NULL immediately after a buffer has been enqueued
  276          */
  277         if (br->br_cons_head == br->br_prod_tail)
  278                 return (NULL);
  279         
  280         return (br->br_ring[br->br_cons_head]);
  281 }
  282 
  283 static __inline int
  284 buf_ring_full(struct buf_ring *br)
  285 {
  286 
  287         return (((br->br_prod_head + 1) & br->br_prod_mask) == br->br_cons_tail);
  288 }
  289 
  290 static __inline int
  291 buf_ring_empty(struct buf_ring *br)
  292 {
  293 
  294         return (br->br_cons_head == br->br_prod_tail);
  295 }
  296 
  297 static __inline int
  298 buf_ring_count(struct buf_ring *br)
  299 {
  300 
  301         return ((br->br_prod_size + br->br_prod_tail - br->br_cons_tail)
  302             & br->br_prod_mask);
  303 }
  304 
  305 struct buf_ring *buf_ring_alloc(int count, struct malloc_type *type, int flags,
  306     struct mtx *);
  307 void buf_ring_free(struct buf_ring *br, struct malloc_type *type);
  308 
  309 
  310 
  311 #endif

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