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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