Well, so my called strictly blocking IO (do and wait till get result before returning from the syscall) only applies on read(). For write(), I agree, kernel may perfer defer the actual write to the physical device so our driver better supports this.
----------------------------------------
> From: oneukum@xxxxxxx
> To: unicorn_wang@xxxxxxxxxxx
> CC: linux-usb@xxxxxxxxxxxxxxx
> Subject: Re: question on skel_read func of usb_skeleton.c
> Date: Fri, 12 Jul 2013 15:38:04 +0200
>
> On Friday 12 July 2013 13:09:23 WangChen wrote:
>> Oliver, my understanding is the limit_sem will only cause write() to sleep and wait for other IOs to finish when there are alreay WRITES_IN_FLIGHT URBs are on going. I see code line 509: sema_init(&dev->limit_sem, WRITES_IN_FLIGHT);
>> My understanding of Blocking IO should block every write() when corresponding URBs is not finished, am I right?
>
> Quoting from the man page of close():
>
> NOTES
> Not checking the return value of close() is a common but nevertheless serious programming error. It is quite possible that errors on a previous write(2) operation are first reported at the final close(). Not checking the return value when closing the file
> may lead to silent loss of data. This can especially be observed with NFS and with disk quota.
>
> A successful close does not guarantee that the data has been successfully saved to disk, as the kernel defers writes. It is not common for a file system to flush the buffers when the stream is closed. If you need to be sure that the data is physically
> stored use fsync(2). (It will depend on the disk hardware at this point.)
>
> It is probably unwise to close file descriptors while they may be in use by system calls in other threads in the same process. Since a file descriptor may be reused, there are some obscure race conditions that may cause unintended side effects.
>
>
> The logic for doing this is implemented in:
>
> static int skel_flush(struct file *file, fl_owner_t id)
> {
> struct usb_skel *dev;
> int res;
>
> dev = file->private_data;
> if (dev == NULL)
> return -ENODEV;
>
> /* wait for io to stop */
> mutex_lock(&dev->io_mutex);
> skel_draw_down(dev);
>
> /* read out errors, leave subsequent opens a clean slate */
> spin_lock_irq(&dev->err_lock);
> res = dev->errors ? (dev->errors == -EPIPE ? -EPIPE : -EIO) : 0;
> dev->errors = 0;
> spin_unlock_irq(&dev->err_lock);
>
> mutex_unlock(&dev->io_mutex);
>
> return res;
> }
>
> Regards
> Oliver
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