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4784a46912
with a new implementation that has a mostly reentrant "addchar" routine, supports multiple message buffers in the kernel, and hides the implementation details from callers. The new code uses a kind of sequence number to represend the current read and write positions in the buffer. This approach (suggested mainly by bde) permits the read and write pointers to be maintained separately, which reduces the number of atomic operations that are required. The "mostly reentrant" above refers to the way that while it is now always safe to have any number of concurrent writers, readers could see the message buffer after a writer has advanced the pointers but before it has witten the new character. Discussed on: freebsd-arch
240 lines
6.4 KiB
C
240 lines
6.4 KiB
C
/*
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* Copyright (c) 2003 Ian Dowse. All rights reserved.
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions
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* are met:
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* 1. Redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer.
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* 2. Redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in the
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* documentation and/or other materials provided with the distribution.
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*
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* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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* ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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* SUCH DAMAGE.
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*
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* $FreeBSD$
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*/
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/*
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* Generic message buffer support routines.
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*/
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#include <sys/param.h>
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#include <sys/systm.h>
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#include <sys/msgbuf.h>
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/* Read/write sequence numbers are modulo a multiple of the buffer size. */
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#define SEQMOD(size) ((size) * 16)
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static u_int msgbuf_cksum(struct msgbuf *mbp);
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/*
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* Initialize a message buffer of the specified size at the specified
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* location. This also zeros the buffer area.
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*/
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void
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msgbuf_init(struct msgbuf *mbp, void *ptr, int size)
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{
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mbp->msg_ptr = ptr;
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mbp->msg_size = size;
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mbp->msg_seqmod = SEQMOD(size);
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msgbuf_clear(mbp);
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mbp->msg_magic = MSG_MAGIC;
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}
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/*
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* Reinitialize a message buffer, retaining its previous contents if
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* the size and checksum are correct. If the old contents cannot be
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* recovered, the message buffer is cleared.
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*/
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void
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msgbuf_reinit(struct msgbuf *mbp, void *ptr, int size)
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{
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u_int cksum;
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if (mbp->msg_magic != MSG_MAGIC || mbp->msg_size != size) {
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msgbuf_init(mbp, ptr, size);
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return;
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}
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mbp->msg_seqmod = SEQMOD(size);
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mbp->msg_wseq = MSGBUF_SEQNORM(mbp, mbp->msg_wseq);
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mbp->msg_rseq = MSGBUF_SEQNORM(mbp, mbp->msg_rseq);
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mbp->msg_ptr = ptr;
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cksum = msgbuf_cksum(mbp);
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if (cksum != mbp->msg_cksum) {
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printf("msgbuf cksum mismatch (read %x, calc %x)\n",
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mbp->msg_cksum, cksum);
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msgbuf_clear(mbp);
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}
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}
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/*
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* Clear the message buffer.
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*/
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void
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msgbuf_clear(struct msgbuf *mbp)
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{
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bzero(mbp->msg_ptr, mbp->msg_size);
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mbp->msg_wseq = 0;
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mbp->msg_rseq = 0;
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mbp->msg_cksum = 0;
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}
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/*
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* Get a count of the number of unread characters in the message buffer.
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*/
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int
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msgbuf_getcount(struct msgbuf *mbp)
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{
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u_int len;
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len = MSGBUF_SEQSUB(mbp, mbp->msg_wseq, mbp->msg_rseq);
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if (len > mbp->msg_size)
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len = mbp->msg_size;
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return (len);
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}
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/*
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* Append a character to a message buffer. This function can be
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* considered fully reentrant so long as the number of concurrent
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* callers is less than the number of characters in the buffer.
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* However, the message buffer is only guaranteed to be consistent
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* for reading when there are no callers in this function.
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*/
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void
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msgbuf_addchar(struct msgbuf *mbp, int c)
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{
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u_int new_seq, pos, seq;
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do {
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seq = mbp->msg_wseq;
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new_seq = MSGBUF_SEQNORM(mbp, seq + 1);
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} while (atomic_cmpset_rel_int(&mbp->msg_wseq, seq, new_seq) == 0);
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pos = MSGBUF_SEQ_TO_POS(mbp, seq);
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atomic_add_int(&mbp->msg_cksum, (u_int)(u_char)c -
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(u_int)(u_char)mbp->msg_ptr[pos]);
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mbp->msg_ptr[pos] = c;
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}
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/*
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* Read and mark as read a character from a message buffer.
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* Returns the character, or -1 if no characters are available.
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*/
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int
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msgbuf_getchar(struct msgbuf *mbp)
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{
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u_int len, wseq;
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int c;
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wseq = mbp->msg_wseq;
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len = MSGBUF_SEQSUB(mbp, wseq, mbp->msg_rseq);
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if (len == 0)
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return (-1);
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if (len > mbp->msg_size)
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mbp->msg_rseq = MSGBUF_SEQNORM(mbp, wseq - mbp->msg_size);
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c = (u_char)mbp->msg_ptr[MSGBUF_SEQ_TO_POS(mbp, mbp->msg_rseq)];
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mbp->msg_rseq = MSGBUF_SEQNORM(mbp, mbp->msg_rseq + 1);
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return (c);
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}
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/*
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* Read and mark as read a number of characters from a message buffer.
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* Returns the number of characters that were placed in `buf'.
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*/
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int
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msgbuf_getbytes(struct msgbuf *mbp, char *buf, int buflen)
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{
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u_int len, pos, wseq;
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wseq = mbp->msg_wseq;
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len = MSGBUF_SEQSUB(mbp, wseq, mbp->msg_rseq);
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if (len == 0)
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return (0);
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if (len > mbp->msg_size) {
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mbp->msg_rseq = MSGBUF_SEQNORM(mbp, wseq - mbp->msg_size);
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len = mbp->msg_size;
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}
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pos = MSGBUF_SEQ_TO_POS(mbp, mbp->msg_rseq);
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len = min(len, mbp->msg_size - pos);
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len = min(len, (u_int)buflen);
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bcopy(&mbp->msg_ptr[pos], buf, len);
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mbp->msg_rseq = MSGBUF_SEQNORM(mbp, mbp->msg_rseq + len);
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return (len);
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}
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/*
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* Peek at the full contents of a message buffer without marking any
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* data as read. `seqp' should point to an unsigned integer that
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* msgbuf_peekbytes() can use to retain state between calls so that
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* the whole message buffer can be read in multiple short reads.
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* To initialise this variable to the start of the message buffer,
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* call msgbuf_peekbytes() with a NULL `buf' parameter.
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*
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* Returns the number of characters that were placed in `buf'.
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*/
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int
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msgbuf_peekbytes(struct msgbuf *mbp, char *buf, int buflen, u_int *seqp)
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{
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u_int len, pos, wseq;
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if (buf == NULL) {
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/* Just initialise *seqp. */
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*seqp = MSGBUF_SEQNORM(mbp, mbp->msg_wseq - mbp->msg_size);
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return (0);
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}
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wseq = mbp->msg_wseq;
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len = MSGBUF_SEQSUB(mbp, wseq, *seqp);
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if (len == 0)
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return (0);
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if (len > mbp->msg_size) {
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*seqp = MSGBUF_SEQNORM(mbp, wseq - mbp->msg_size);
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len = mbp->msg_size;
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}
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pos = MSGBUF_SEQ_TO_POS(mbp, *seqp);
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len = min(len, mbp->msg_size - pos);
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len = min(len, (u_int)buflen);
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bcopy(&mbp->msg_ptr[MSGBUF_SEQ_TO_POS(mbp, *seqp)], buf, len);
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*seqp = MSGBUF_SEQNORM(mbp, *seqp + len);
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return (len);
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}
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/*
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* Compute the checksum for the complete message buffer contents.
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*/
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static u_int
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msgbuf_cksum(struct msgbuf *mbp)
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{
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u_int i, sum;
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sum = 0;
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for (i = 0; i < mbp->msg_size; i++)
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sum += (u_char)mbp->msg_ptr[i];
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return (sum);
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}
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/*
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* Copy from one message buffer to another.
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*/
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void
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msgbuf_copy(struct msgbuf *src, struct msgbuf *dst)
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{
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int c;
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while ((c = msgbuf_getchar(src)) >= 0)
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msgbuf_addchar(dst, c);
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}
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