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/9.1/sys/sys/buf_ring.h 207673 2010-05-05 20:39:02Z joel $
   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         uint64_t                br_prod_bufs;
   51         uint64_t                br_prod_bytes;
   52         /*
   53          * Pad out to next L2 cache line
   54          */
   55         uint64_t                _pad0[11];
   56 
   57         volatile uint32_t       br_cons_head;
   58         volatile uint32_t       br_cons_tail;
   59         int                     br_cons_size;
   60         int                     br_cons_mask;
   61         
   62         /*
   63          * Pad out to next L2 cache line
   64          */
   65         uint64_t                _pad1[14];
   66 #ifdef DEBUG_BUFRING
   67         struct mtx              *br_lock;
   68 #endif  
   69         void                    *br_ring[0];
   70 };
   71 
   72 /*
   73  * multi-producer safe lock-free ring buffer enqueue
   74  *
   75  */
   76 static __inline int
   77 buf_ring_enqueue_bytes(struct buf_ring *br, void *buf, int nbytes)
   78 {
   79         uint32_t prod_head, prod_next;
   80         uint32_t cons_tail;
   81         int success;
   82 #ifdef DEBUG_BUFRING
   83         int i;
   84         for (i = br->br_cons_head; i != br->br_prod_head;
   85              i = ((i + 1) & br->br_cons_mask))
   86                 if(br->br_ring[i] == buf)
   87                         panic("buf=%p already enqueue at %d prod=%d cons=%d",
   88                             buf, i, br->br_prod_tail, br->br_cons_tail);
   89 #endif  
   90         critical_enter();
   91         do {
   92                 prod_head = br->br_prod_head;
   93                 cons_tail = br->br_cons_tail;
   94 
   95                 prod_next = (prod_head + 1) & br->br_prod_mask;
   96                 
   97                 if (prod_next == cons_tail) {
   98                         critical_exit();
   99                         return (ENOBUFS);
  100                 }
  101                 
  102                 success = atomic_cmpset_int(&br->br_prod_head, prod_head,
  103                     prod_next);
  104         } while (success == 0);
  105 #ifdef DEBUG_BUFRING
  106         if (br->br_ring[prod_head] != NULL)
  107                 panic("dangling value in enqueue");
  108 #endif  
  109         br->br_ring[prod_head] = buf;
  110         wmb();
  111 
  112         /*
  113          * If there are other enqueues in progress
  114          * that preceeded us, we need to wait for them
  115          * to complete 
  116          */   
  117         while (br->br_prod_tail != prod_head)
  118                 cpu_spinwait();
  119         br->br_prod_bufs++;
  120         br->br_prod_bytes += nbytes;
  121         br->br_prod_tail = prod_next;
  122         critical_exit();
  123         return (0);
  124 }
  125 
  126 static __inline int
  127 buf_ring_enqueue(struct buf_ring *br, void *buf)
  128 {
  129 
  130         return (buf_ring_enqueue_bytes(br, buf, 0));
  131 }
  132 
  133 /*
  134  * multi-consumer safe dequeue 
  135  *
  136  */
  137 static __inline void *
  138 buf_ring_dequeue_mc(struct buf_ring *br)
  139 {
  140         uint32_t cons_head, cons_next;
  141         uint32_t prod_tail;
  142         void *buf;
  143         int success;
  144 
  145         critical_enter();
  146         do {
  147                 cons_head = br->br_cons_head;
  148                 prod_tail = br->br_prod_tail;
  149 
  150                 cons_next = (cons_head + 1) & br->br_cons_mask;
  151                 
  152                 if (cons_head == prod_tail) {
  153                         critical_exit();
  154                         return (NULL);
  155                 }
  156                 
  157                 success = atomic_cmpset_int(&br->br_cons_head, cons_head,
  158                     cons_next);
  159         } while (success == 0);         
  160 
  161         buf = br->br_ring[cons_head];
  162 #ifdef DEBUG_BUFRING
  163         br->br_ring[cons_head] = NULL;
  164 #endif
  165         rmb();
  166         
  167         /*
  168          * If there are other dequeues in progress
  169          * that preceeded us, we need to wait for them
  170          * to complete 
  171          */   
  172         while (br->br_cons_tail != cons_head)
  173                 cpu_spinwait();
  174 
  175         br->br_cons_tail = cons_next;
  176         critical_exit();
  177 
  178         return (buf);
  179 }
  180 
  181 /*
  182  * single-consumer dequeue 
  183  * use where dequeue is protected by a lock
  184  * e.g. a network driver's tx queue lock
  185  */
  186 static __inline void *
  187 buf_ring_dequeue_sc(struct buf_ring *br)
  188 {
  189         uint32_t cons_head, cons_next, cons_next_next;
  190         uint32_t prod_tail;
  191         void *buf;
  192         
  193         cons_head = br->br_cons_head;
  194         prod_tail = br->br_prod_tail;
  195         
  196         cons_next = (cons_head + 1) & br->br_cons_mask;
  197         cons_next_next = (cons_head + 2) & br->br_cons_mask;
  198         
  199         if (cons_head == prod_tail) 
  200                 return (NULL);
  201 
  202 #ifdef PREFETCH_DEFINED 
  203         if (cons_next != prod_tail) {           
  204                 prefetch(br->br_ring[cons_next]);
  205                 if (cons_next_next != prod_tail) 
  206                         prefetch(br->br_ring[cons_next_next]);
  207         }
  208 #endif
  209         br->br_cons_head = cons_next;
  210         buf = br->br_ring[cons_head];
  211 
  212 #ifdef DEBUG_BUFRING
  213         br->br_ring[cons_head] = NULL;
  214         if (!mtx_owned(br->br_lock))
  215                 panic("lock not held on single consumer dequeue");
  216         if (br->br_cons_tail != cons_head)
  217                 panic("inconsistent list cons_tail=%d cons_head=%d",
  218                     br->br_cons_tail, cons_head);
  219 #endif
  220         br->br_cons_tail = cons_next;
  221         return (buf);
  222 }
  223 
  224 /*
  225  * return a pointer to the first entry in the ring
  226  * without modifying it, or NULL if the ring is empty
  227  * race-prone if not protected by a lock
  228  */
  229 static __inline void *
  230 buf_ring_peek(struct buf_ring *br)
  231 {
  232 
  233 #ifdef DEBUG_BUFRING
  234         if ((br->br_lock != NULL) && !mtx_owned(br->br_lock))
  235                 panic("lock not held on single consumer dequeue");
  236 #endif  
  237         /*
  238          * I believe it is safe to not have a memory barrier
  239          * here because we control cons and tail is worst case
  240          * a lagging indicator so we worst case we might
  241          * return NULL immediately after a buffer has been enqueued
  242          */
  243         if (br->br_cons_head == br->br_prod_tail)
  244                 return (NULL);
  245         
  246         return (br->br_ring[br->br_cons_head]);
  247 }
  248 
  249 static __inline int
  250 buf_ring_full(struct buf_ring *br)
  251 {
  252 
  253         return (((br->br_prod_head + 1) & br->br_prod_mask) == br->br_cons_tail);
  254 }
  255 
  256 static __inline int
  257 buf_ring_empty(struct buf_ring *br)
  258 {
  259 
  260         return (br->br_cons_head == br->br_prod_tail);
  261 }
  262 
  263 static __inline int
  264 buf_ring_count(struct buf_ring *br)
  265 {
  266 
  267         return ((br->br_prod_size + br->br_prod_tail - br->br_cons_tail)
  268             & br->br_prod_mask);
  269 }
  270 
  271 struct buf_ring *buf_ring_alloc(int count, struct malloc_type *type, int flags,
  272     struct mtx *);
  273 void buf_ring_free(struct buf_ring *br, struct malloc_type *type);
  274 
  275 
  276 
  277 #endif

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