c - mmap/munmap 问题 - 在第 783 次迭代后出现总线错误？

Question

好的，这是设置:我在 HPC 工作，我们正在为扩展到数万个节点的需求做准备。为了解决这个问题，我实现了一个本地进程，在每个节点上缓存信息以减少网络流量。然后它通过共享内存公开此信息。基本逻辑是有一个众所周知的共享内存块，其中包含当前缓存表的名称。当更新发生时，缓存工具创建一个新的共享内存表，填充它，然后用新表的名称更新众所周知的 block 。

代码似乎可以正常工作(例如，valgrind 说没有内存泄漏)但是当我故意对其进行压力测试时，前 783 次更新工作得非常好 - 但在第 784 次，当我尝试更新时出现 SIGBUS 错误写入映射内存。

如果问题是打开的文件太多(因为我正在泄漏文件描述符)，我预计 shm_open() 会失败。如果问题是我正在泄漏映射内存，我希望 mmap() 失败或 valgrind 报告泄漏。

这是代码片段。谁能给个建议？

int
initialize_paths(writer_t *w, unsigned max_paths)
{
int err = 0;
reader_t *r = &(w->unpublished);

close_table(r,PATH_TABLE);

w->max_paths = max_paths;

err = open_table(r, PATH_TABLE, O_RDWR | O_CREAT, max_paths, 0);

return err;
}

static void
close_table(reader_t *r, int table)
{
    if (r->path_table && r->path_table != MAP_FAILED) {
       munmap(r->path_table,r->path_table->size);
       r->path_table=NULL;
    }
    if (r->path_fd>0) { close(r->path_fd); r->path_fd=0; }
}


static int
open_table(op_ppath_reader_t *r, int table, int rw, unsigned c, unsigned c2)
{
// Code omitted for clarity
if (rw & O_CREAT) {
    prot = PROT_READ | PROT_WRITE;
} else {
    // Note that this overrides the sizes set above.
    // We will get the real sizes from the header.
    prot = PROT_READ;
    size1 = sizeof(op_ppath_header_t);
    size2 = 0;
}

fd = shm_open(name, rw, 0644);
if (fd < 0) {
    _DBG_ERROR("Failed to open %s\n",name);
    goto error;
}

if (rw & O_CREAT) {
    /* Create the file at the specified size. */
    if (ftruncate(fd, size1 + size2)) {
        _DBG_ERROR("Unable to size %s\n",name);
        goto error;
    }
}

h = (op_ppath_header_t*)mmap(0, size1 + size2, prot, MAP_SHARED, fd, 0);
if (h == MAP_FAILED) {
    _DBG_ERROR("Unable to map %s\n",name);
    goto error;
}

if (rw & O_CREAT) {
    /*
     * clear the table & set the maximum lengths.
     */
    memset((char*)h,0,size1+size2);  -- SIGBUS OCCURS HERE
    h->s1 = size1;
    h->s2 = size2;
} else {
// more code omitted for clarity.
}

更新:

下面是一些失败的示例调试输出:

NOTICE: Pass 783: Inserting records.
NOTICE: Creating the path table.
TRC: initialize_paths[
TRC: close_table[
TRC: close_table]
TRC: open_table[
DBG: h=0x0x2a956b2000, size1=2621536, size2=0

这是上一次迭代的相同输出:

NOTICE: Pass 782: Inserting records.
NOTICE: Creating the path table.
TRC: initialize_paths[
TRC: close_table[
TRC: close_table]
TRC: open_ppath_table[
DBG: h=0x0x2a956b2000, size1=2621536, size2=0
TRC: open_ppath_table]
TRC: op_ppath_initialize_paths]

请注意，指针地址是有效的，大小也是有效的。

GDB 以这种方式报告崩溃:

Program received signal SIGBUS, Bus error.
[Switching to Thread 182895447776 (LWP 5328)]
0x00000034a9371d20 in memset () from /lib64/tls/libc.so.6
(gdb) where
#0  0x00000034a9371d20 in memset () from /lib64/tls/libc.so.6
#1  0x0000002a955949d0 in open_table (r=0x7fbffff188, table=1, rw=66,
c=32768, c2=0) at ofedplus_path_private.c:294
#2  0x0000002a95595280 in initialize_paths (w=0x7fbffff130,
    max_paths=32768) at path_private.c:567
#3  0x0000000000402050 in server (fname=0x7fbffff270 "gidtable", n=10000)
    at opp_cache_test.c:202
#4  0x0000000000403086 in main (argc=6, argv=0x7fbffff568)
    at opp_cache_test.c:542

(gdb)

当在下一行设置 h->size1 并且 size1 是映射区域的前 4 个字节时，删除 memset 仍然会导致 SIGBUS。

Answer 1

SIGBUS 可能是由对您的 SHM 对象的多次引用引起的。
查看上面的代码，您使用了 shm_open()、mmap()、munmap() 但是
你缺少 shm_unlink()。

正如 *shm_open/shm_close 的联机帮助页所述，这些对象是引用计数的。

The operation of shm_unlink is analogous to unlink(2): it removes a shared memory object name, and, once all processes have unmapped the object, deallocates and destroys the contents of the associated memory region.
After a successful shm_unlink, attempts to shm_open an object with the same name will fail (unless O_CREAT was specified, in which case a new, distinct object is created).

也许这些信息将有助于解决您的问题。