rdsbufalloc.c

Go to the documentation of this file.

/* rdsbufalloc.c - rdsbufalloc */

#include <xinu.h>

/*------------------------------------------------------------------------
 * rdsbufalloc  -  Allocate a buffer from the free list or the cache
 *------------------------------------------------------------------------
 */
struct rdbuff *rdsbufalloc (
      struct rdscblk *rdptr     /* Ptr to device control block  */
    )
{
    struct  rdbuff  *bptr;      /* Pointer to a buffer      */
    struct  rdbuff  *pptr;      /* Pointer to previous buffer   */
    struct  rdbuff  *nptr;      /* Pointer to next buffer   */

    /* Wait for an available buffer */

    wait(rdptr->rd_availsem);

    /* If free list contains a buffer, extract it */

    bptr = rdptr->rd_free;

    if ( bptr != (struct rdbuff *)NULL ) {
        rdptr->rd_free = bptr->rd_next;
        return bptr;
    }

    /* Extract oldest item in cache that has ref count zero (at */
    /*   least one such entry must exist because the semaphore  */
    /*   had a nonzero count)                   */

    bptr = rdptr->rd_ctprev;
    while (bptr != (struct rdbuff *) &rdptr->rd_chnext) {
        if (bptr->rd_refcnt <= 0) {

            /* Remove from cache and return to caller */

            pptr = bptr->rd_prev;
            nptr = bptr->rd_next;
            pptr->rd_next = nptr;
            nptr->rd_prev = pptr;
            return bptr;
        }
        bptr = bptr->rd_prev;
    }
    panic("Remote disk cannot find an available buffer");
    return (struct rdbuff *)SYSERR;
}