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?
----------------------------------------
> 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 10:30:28 +0200
>
> On Friday 12 July 2013 04:49:54 WangChen wrote:
>> Hi, Oliver,
>> Regarding skel_write, I see your current desgin only refuse its execution when> WRITES_IN_FLIGHT are on the fly, but this is not blocking IO due to write() will not wait before callback returns, right? Do you think it's unnecessary to support blocking IO on write or any special reason else?
>
>
> /*
> * limit the number of URBs in flight to stop a user from using up all
> * RAM
> */
> if (!(file->f_flags & O_NONBLOCK)) {
> if (down_interruptible(&dev->limit_sem)) {
>
> The limit_sem will cause write() to wait for IO to finish,
> if the limit is reached
>
> retval = -ERESTARTSYS;
> goto exit;
> }
> } else {
> if (down_trylock(&dev->limit_sem)) {
> retval = -EAGAIN;
>
> Unless non-blocking IO is used. In that case we return.
>
> goto exit;
> }
> }
>
> Blocking IO is perfectly well supported. To keep under the limit you don't
> have to wait for the IO you started. Any IO will serve the purpose.
>
> Regards
> Oliver
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