linux-kernel - Re: Memory barrier needed with wake_up

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Message-ID: <87mveq95jb.fsf@linux.intel.com>
Date:   Mon, 16 Jan 2017 21:19:20 +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,

Felipe Balbi <felipe.balbi@...ux.intel.com> writes:
> 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 ;-)

*yet* is a keyword here. I wouldn't call this problem "done" until I
 successfully run my test case for at least a week.

-- 
balbi