file->f_pos lock?

[Vasiliy Kulikov <segooon@xxxxxxxxx>]

Hi,

I wonder how file->f_pos is protected in read()/lseek()/etc.


Code from fs/read_write.c:

    static inline loff_t file_pos_read(struct file *file)
    {
        return file->f_pos;
    }

    static inline void file_pos_write(struct file *file, loff_t pos)
    {
        file->f_pos = pos;
    }

    SYSCALL_DEFINE3(read, unsigned int, fd, char __user *, buf, size_t, count)
    {
        ...
        if (file) {
            loff_t pos = file_pos_read(file);
                         ^^^^^^^^^^^^^
            ret = vfs_read(file, buf, count, &pos);
            file_pos_write(file, pos);
            ^^^^^^^^^^^^^^
            fput_light(file, fput_needed);
        }
        ...
    }

Update of file->f_pos in sys_lseek() is fully done in driver defined lseek().


I don't understand one thing: if driver lseek() is even protected with
lock_kernel() how does it protect f_pos from changing from read()? E.g.


sys_read()
...
pos = file_pos_read()
... /* update pos */
...                     sys_lseek()
...                     {
...                       file->f_pos = new_pos;
...                     }

file_pos_write(file, pos)
^^^^^^^^^^^^^^^^^^^^^^^^^

file_pos_write() does overwrites current position with old value,
without dependence of lock style.



Also if driver uses generic_file_llseek() does read()/write()/etc need to lock
file->f_dentry->d_inode->i_mutex? I don't see it in some drivers, e.g. in 
drivers/hid/hid-picolcd.c.


Thanks in advance,
Vasiliy

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