Oliver,
For the func skel_read(), line 328~330, for the case rv ==0, can we just do retry, similar as the code line 301. I hope this can help read() get data in one time for blocking IO if the data to be read is short and can be received in quick time. The existing retry logic upon should be able to handle all the other stuff such as nonblockking IO in one place.
----------------------------------------
> From: unicorn_wang@xxxxxxxxxxx
> To: oneukum@xxxxxxx
> CC: linux-usb@xxxxxxxxxxxxxxx
> Subject: RE: question on skel_read func of usb_skeleton.c
> Date: Sun, 14 Jul 2013 08:48:52 +0000
>
> 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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