linux-kernel - Re: Memory barrier needed with wake_up

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Date:   Mon, 16 Jan 2017 21:04:49 +0200
From:   Felipe Balbi <felipe.balbi@...ux.intel.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...hat.com>,
        USB list <linux-usb@...r.kernel.org>,
        Kernel development list <linux-kernel@...r.kernel.org>,
        Will Deacon <will.deacon@....com>
Subject: Re: Memory barrier needed with wake_up_process()?


Hi,

Alan Stern <stern@...land.harvard.edu> writes:
> 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?

couldn't reproduce the problem yet ;-)

-- 
balbi