linux-kernel - Re: [PATCH] ohci-hcd: Fix race condition caused by ohci_urb_enqueue() and io_watchdog

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Date:   Fri, 2 Feb 2018 01:46:01 +0000
From:   "Yoshida, Shigeru" <Shigeru.Yoshida@...driver.com>
To:     "stern@...land.harvard.edu" <stern@...land.harvard.edu>
CC:     "Bai, Haiqing" <Haiqing.Bai@...driver.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ohci-hcd: Fix race condition caused by
 ohci_urb_enqueue() and io_watchdog_func()

Hi Alan,

On Thu, 1 Feb 2018 10:33:33 -0500, Alan Stern wrote:
>> I think we must choose an invalid frame number for the special
>> sentinel value, but I'm not sure which value is adequate for it.
>> Is 0xffffff00 an invalid frame number, otherwise how about simply
>> -1(0xffffffff)?
> 
> Well, the frame_no register is 32 bits wide, but only the 16 low-order 
> bits are meaningful.  ohci_frame_no() strips off the high-order 16 
> bits, so any value with one of those bits set would be acceptable.  
> (Besides, valid frame numbers only go up to 2047.)
> 
> I chose 0xffffff00 because PCI reads from a non-working device 
> generally get a value with all the bits set.  But since the upper 16 
> bits are masked away anyhow, it doesn't matter.  -1u would be fine.

Thanks, I choose 0xffffff00 as you suggested, and will prepare v2 patch.

Thanks,
Shigeru