很久都没有写驱动代码了,对于一些驱动相关的内核变化也没有怎么关心。这次重游《LDD3》获益良多,其值对于struct file_operations中ioctl的消失也让我长了不少见识。
当年看《LDD3》的时候已经注意到了书中对ioctl的评价不是很好:“ioctl调用的非结构化本质导致众多内核开发者倾向于放弃它。” ,而在这次阅读3.0代码的时候,这个成员在struct file_operations中早已消失了。这个激起了我学习的兴趣,以下是对这个ioctl的学习小结:
1、消失的确切时间
ioctl的消失到底是从哪个版本开始的?网上给出的时间是2.6.36开始。网上就是这么说,但是自己必须找到代码中的证据。于是我通过git搜索主线内核代码,找到的删除ioctl的那个提交:
- commit b19dd42faf413b4705d4adb38521e82d73fa4249
-
Author: Arnd Bergmann
-
Date: Sun Jul 4 00:15:10 2010 +0200
-
bkl: Remove locked .ioctl file operation
-
The last user is gone, so we can safely remove this
-
Signed-off-by: Arnd Bergmann
-
Cc: John Kacur
-
Cc: Al Viro
-
Cc: Thomas Gleixner
-
Signed-off-by: Frederic Weisbecker
好不容易找到了这个提交,好的,这样就可以确定消失的时间了:
- git tag --contains b19dd42
-
v2.6.36
-
v2.6.36-rc1
-
v2.6.36-rc2
-
v2.6.36-rc3
-
v2.6.36-rc4
-
v2.6.36-rc5
-
v2.6.36-rc6
-
v2.6.36-rc7
-
v2.6.36-rc8
-
......以下省略ooxx行
可以证明ioctl消失的版本是v2.6.35到v2.6.36-rc1间,于是我导出了v2.6.35到v2.6.36-rc1的补丁,果真在其中!
git diff v2.6.35..v2.6.36-rc1 >
补丁过大不易上传,请自行生成。
2、消失的原因
简单的概括:这次ioctl的消失,并不是要把ioctl清理出去,而是要逐步的清理大内核锁(BKL)。
这个让ioctl消失的过渡期长达5年,从2005年开始内核黑客就开始替换ioctl了。具体的原因在中有一篇很好的文章:。我将他翻译了一下:ioctl()的新方法(必看)
当然,顺便了解一下大内核锁也是很有必要的:转载好文:《大内核锁将何去何从》
3、ioctl的替代者
对于原来的ioctl,其实可以叫做locked ioctl。这个其实是相对于他的替代方法来讲的。我们来看看2.6.35以前在struct file_operations中有关ioctl的成员:
- /*
-
* NOTE:
-
* read, write, poll, fsync, readv, writev, unlocked_ioctl and compat_ioctl
-
* can be called without the big kernel lock held in all filesystems.
-
*/
-
struct file_operations {
-
struct module *owner;
-
loff_t (*llseek) (struct file *, loff_t, int);
-
ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
-
ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
-
ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
-
ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
-
int (*readdir) (struct file *, void *, filldir_t);
-
unsigned int (*poll) (struct file *, struct poll_table_struct *);
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int (*ioctl) (struct inode *, struct file *, unsigned int, unsigned long);
-
long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
-
long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
-
int (*mmap) (struct file *, struct vm_area_struct *);
-
int (*open) (struct inode *, struct file *);
-
int (*flush) (struct file *, fl_owner_t id);
-
int (*release) (struct inode *, struct file *);
-
int (*fsync) (struct file *, struct dentry *, int datasync);
-
int (*aio_fsync) (struct kiocb *, int datasync);
-
int (*fasync) (int, struct file *, int);
-
int (*lock) (struct file *, int, struct file_lock *);
-
ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
-
unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
-
int (*check_flags)(int);
-
int (*flock) (struct file *, int, struct file_lock *);
-
ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
-
ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
-
int (*setlease)(struct file *, long, struct file_lock **);
-
};
这个结构体其实是在过渡期的结构体,unlocked_ioctl就是ioctl的替代者。对于新的驱动,不要再使用ioctl了,而是使用unlocked_ioctl。
4、调用ioctl与unlocked_ioctl在内核代码上的不同
其实ioctl与unlocked_ioctl所对应的系统调用都是ioctl。但是在应用层调用ioctl的时候,对于我们实现ioctl或者unlocked_ioctl有什么不同呢?这里我们可以追溯一下ioctl系统调用代码的执行过程,这里我简单的写出这个系统调用对于设备驱动(一般是设备驱动使用ioctl)的执行顺序:(fs/ioctl.c)
SYSCALL_DEFINE3(ioctl, unsigned int, fd, unsigned int, cmd, unsigned long, arg)--->do_vfs_ioctl---> vfs_ioctl
而ioctl与unlocked_ioctl的区别就体现在了这个vfs_ioctl中,我们先来看ioctl被删除前的函数:
- /**
-
* vfs_ioctl - call filesystem specific ioctl methods
-
* @filp: open file to invoke ioctl method on
-
* @cmd: ioctl command to execute
-
* @arg: command-specific argument for ioctl
-
*
-
* Invokes filesystem specific ->unlocked_ioctl, if one exists; otherwise
-
* invokes filesystem specific ->ioctl method. If neither method exists,
-
* returns -ENOTTY.
-
*
-
* Returns 0 on success, -errno on error.
-
*/
-
static long vfs_ioctl(struct file *filp, unsigned int cmd,
-
unsigned long arg)
-
{
-
int error = -ENOTTY;
-
-
if (!filp->f_op)
-
goto out;
-
-
if (filp->f_op->unlocked_ioctl) {
-
error = filp->f_op->unlocked_ioctl(filp, cmd, arg);
-
if (error == -ENOIOCTLCMD)
-
error = -EINVAL;
-
goto out;
-
} else if (filp->f_op->ioctl) {
-
lock_kernel();
-
error = filp->f_op->ioctl(filp->f_path.dentry->d_inode,
-
filp, cmd, arg);
-
unlock_kernel();
-
}
-
-
out:
-
return error;
-
}
从这个函数中我们可以看出:
- ioctl是受到大内核锁保护的,而unlocked_ioctl是直接执行的。
- unlocked_ioctl优先级高于ioctl,如果存在unlocked_ioctl,则执行unlocked_ioctl,否则才执行ioctl。这个优先级的产生明显是为了过渡。
而在ioctl被删除后,vfs_ioctl函数也做了相应的改变(Linux-3.0):
- /**
-
* vfs_ioctl - call filesystem specific ioctl methods
-
* @filp: open file to invoke ioctl method on
-
* @cmd: ioctl command to execute
-
* @arg: command-specific argument for ioctl
-
*
-
* Invokes filesystem specific ->unlocked_ioctl, if one exists; otherwise
-
* returns -ENOTTY.
-
*
-
* Returns 0 on success, -errno on error.
-
*/
-
static long vfs_ioctl(struct file *filp, unsigned int cmd,
-
unsigned long arg)
-
{
-
int error = -ENOTTY;
-
-
if (!filp->f_op || !filp->f_op->unlocked_ioctl)
-
goto out;
-
-
error = filp->f_op->unlocked_ioctl(filp, cmd, arg);
-
if (error == -ENOIOCTLCMD)
-
error = -EINVAL;
-
out:
-
return error;
-
}
5、在驱动编程时的注意事项
- 在注册文件操作方法的结构体
struct file_operations的时候原先的.ioctl=OOXX;替换为 .unlocked_ioctl=OOXX;
但是要注意ioctl和unlocked_ioctl的定义有一点不同:unlocked_ioctl少了一个inode参数。但是如果方法中真的需要其中的数据,可以通过filp->f_dentry->d_inode获得。- 由于失去了大内核锁的保护,所以必须在unlocked_ioctl方法中自行实现锁机制,以保证不会在操作设备的时候(特别在SMP系统中)产生竞态。(也就实现了用小锁替换大锁)
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