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

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Message-ID: <551AED33.2040206@mentor.com>
Date:	Tue, 31 Mar 2015 19:53:39 +0100
From:	"Baxter, Jim" <jim_baxter@...tor.com>
To:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>
CC:	"Zapolskiy, Vladimir" <Vladimir_Zapolskiy@...tor.com>
Subject: usb: gadget: f_fs: O_NONBLOCK waits MAX_SCHEDULE_TIMEOUT

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?

Thank you,
Jim
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