内核保留内存防止泄漏的机制_转

转自：http://blog.csdn.net/adaptiver/article/details/7425496

保留内存防止泄漏的机制：进程退出会调用
do_exit->exit_files->put_files_struct->close_files->filp_close->fput->__fput
在__fput里调用file->f_op->release(inode, file)
而在保留内存对应release函数中，根据tgid做出判断，如果有对应tgid的保留内存块没有被主动释放，则认为是进程非正常退出，测试释放与tgid对应保留内存块。
但在android中调试发现，binder有时会在binder_deferred_func函数中调用put_files_struct来释放某个进程所属的文件资源，此时由于tgid属于binder所在workqueue所在的thread，保留内存检测不到该tgid，即使原来的进程有申请保留内存也不会被释放，从而出现泄漏。

内核中binder在binder_init中建立了一个workqueue

 binder_deferred_workqueue = create_singlethread_workqueue("binder");

void put_files_struct(struct files_struct *files)
{
 struct fdtable *fdt;

 if (atomic_dec_and_test(&files->count)) {
  close_files(files);
  /*
   * Free the fd and fdset arrays if we expanded them.
   * If the fdtable was embedded, pass files for freeing
   * at the end of the RCU grace period. Otherwise,
   * you can free files immediately.
   */
  rcu_read_lock();
  fdt = files_fdtable(files);
  if (fdt != &files->fdtab)
   kmem_cache_free(files_cachep, files);
  free_fdtable(fdt);
  rcu_read_unlock();
 }
}

static void close_files(struct files_struct * files)
{
 int i, j;
 struct fdtable *fdt;

 j = 0;

 /*
  * It is safe to dereference the fd table without RCU or
  * ->file_lock because this is the last reference to the
  * files structure.  But use RCU to shut RCU-lockdep up.
  */
 rcu_read_lock();
 fdt = files_fdtable(files);
 rcu_read_unlock();
 for (;;) {
  unsigned long set;
  i = j * __NFDBITS;
  if (i >= fdt->max_fds)
   break;
  set = fdt->open_fds->fds_bits[j++];
  while (set) {
   if (set & 1) {
    struct file * file = xchg(&fdt->fd[i], NULL);
    if (file) {
     filp_close(file, files);
     cond_resched();
    }
   }
   i++;
   set >>= 1;
  }
 }
}

int filp_close(struct file *filp, fl_owner_t id)
{
 int retval = 0;

 if (!file_count(filp)) {
  printk(KERN_ERR "VFS: Close: file count is 0 ");
  return 0;
 }

 if (filp->f_op && filp->f_op->flush)
  retval = filp->f_op->flush(filp, id);

 dnotify_flush(filp, id);
 locks_remove_posix(filp, id);
 fput(filp);
 return retval;
}

void fput(struct file *file)
{
 if (atomic_long_dec_and_test(&file->f_count))
  __fput(file);
}

static void __fput(struct file *file)
{
 struct dentry *dentry = file->f_path.dentry;
 struct vfsmount *mnt = file->f_path.mnt;
 struct inode *inode = dentry->d_inode;

 might_sleep();

 fsnotify_close(file);
 /*
  * The function eventpoll_release() should be the first called
  * in the file cleanup chain.
  */
 eventpoll_release(file);
 locks_remove_flock(file);

 if (unlikely(file->f_flags & FASYNC)) {
  if (file->f_op && file->f_op->fasync)
   file->f_op->fasync(-1, file, 0);
 }
 if (file->f_op && file->f_op->release)
  file->f_op->release(inode, file);
 security_file_free(file);
 ima_file_free(file);
 if (unlikely(S_ISCHR(inode->i_mode) && inode->i_cdev != NULL))
  cdev_put(inode->i_cdev);
 fops_put(file->f_op);
 put_pid(file->f_owner.pid);
 file_kill(file);
 if (file->f_mode & FMODE_WRITE)
  drop_file_write_access(file);
 file->f_path.dentry = NULL;
 file->f_path.mnt = NULL;
 file_free(file);
 dput(dentry);
 mntput(mnt);
}

static void binder_deferred_func(struct work_struct *work)
{
 struct binder_proc *proc;
 struct files_struct *files;

 int defer;
 do {
  mutex_lock(&binder_lock);
  mutex_lock(&binder_deferred_lock);
  if (!hlist_empty(&binder_deferred_list)) {
   proc = hlist_entry(binder_deferred_list.first,
     struct binder_proc, deferred_work_node);
   hlist_del_init(&proc->deferred_work_node);
   defer = proc->deferred_work;
   proc->deferred_work = 0;
  } else {
   proc = NULL;
   defer = 0;
  }
  mutex_unlock(&binder_deferred_lock);

  files = NULL;
  if (defer & BINDER_DEFERRED_PUT_FILES) {
   files = proc->files;
   if (files)
    proc->files = NULL;
  }

  if (defer & BINDER_DEFERRED_FLUSH)
   binder_deferred_flush(proc);

  if (defer & BINDER_DEFERRED_RELEASE)
   binder_deferred_release(proc); /* frees proc */

  mutex_unlock(&binder_lock);
  if (files)
   put_files_struct(files);
 } while (proc);
}