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Date:   Mon, 16 Jan 2017 12:09:20 -0500 (EST)
From:   Alan Stern <>
To:     Felipe Balbi <>
cc:     Peter Zijlstra <>,
        "Paul E. McKenney" <>,
        Ingo Molnar <>,
        USB list <>,
        Kernel development list <>,
        Will Deacon <>
Subject: Re: Memory barrier needed with wake_up_process()?

On Mon, 16 Jan 2017, Felipe Balbi wrote:

> Sorry for the long delay, I finally have more information on this. All
> this time I was doing something that I never considered to matter: I've
> been running host and peripheral on the same machine. Now that I have
> tracepoints on xHCI as well, I could see that these 30 seconds of
> "nothing" is actuall full of xHCI activity and I can see that for the
> duration of these 30 seconds preempt depth on the CPU that (eventually)
> queues a request on dwc3, is always > 1 (sometimes 2, most of the time
> 1). My conclusion from that is that xHCI (or usbcore ?!?) locks the CPU
> and g_mass_storage is spinning for over 30 seconds at which point
> storage.ko (host side class driver) dequeues the request.
> I'll see if I can capture a fresh trace with both xHCI and dwc3 with
> this happening, but probably not today (testing stuff for -rc).

Does anything change if the host and peripheral are separate machines?

Alan Stern

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