linux-kernel - Re: usb: gadget: f_fs: O_NONBLOCK waits MAX_SCHEDULE

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Date:	Wed, 01 Apr 2015 10:06:35 +0200
From:	Robert Baldyga <r.baldyga@...sung.com>
To:	"Baxter, Jim" <jim_baxter@...tor.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"Zapolskiy, Vladimir" <Vladimir_Zapolskiy@...tor.com>,
	"balbi@...com" <balbi@...com>
Subject: Re: usb: gadget: f_fs: O_NONBLOCK waits MAX_SCHEDULE_TIMEOUT

Hi,

On 03/31/2015 08:53 PM, Baxter, Jim wrote:
> Hi,
> 
> I have been looking at an issue where a phone that is the Function FS
> host sometimes locks up and causes the function:
> static ssize_t ffs_epfile_io(struct file *file, struct ffs_io_data
> *io_data) in drivers/usb/gadget/function/f_fs.c to timeout after
> MAX_SCHEDULE_TIMEOUT jiffies.
> 
> We are opening the ffs connection with the O_NONBLOCK option and this
> timeout seems to be far too long.
> 
> The code causing the wait is here (line 834):
> 
> if (unlikely(ret < 0)) {
>     /* nop */
> } else if (unlikely(
>         wait_for_completion_interruptible(&done))) {
>     ret = -EINTR;
>     usb_ep_dequeue(ep->ep, req);
> } else {
> 
> The function call wait_for_completion_interruptible(&done) equates to:
> wait_for_completion_interruptible_timeout(&done, MAX_SCHEDULE_TIMEOUT);
> 
> 
> I was wondering what to change to fix this, is it reasonable to use a
> very small timeout if it is O_NONBLOCK or is any blocking not allowed?
> 
> I would appreciate any thoughts on how this should work?

FunctionFS is very specific, because read/write operations are directly
translated into USB requests, which are asynchronous, so you cannot use
O_NONBLOCK.

If you need non-blocking API you can use Asynchronous I/O (AIO). You can
find some examples in kernel sources (tools/usb/ffs-aio-example/).

Br,
Robert Baldyga
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