From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1754148AbcICMbn (ORCPT <rfc822;w@1wt.eu>);
        Sat, 3 Sep 2016 08:31:43 -0400
Received: from bombadil.infradead.org ([198.137.202.9]:49082 "EHLO
        bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1752434AbcICMbj (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Sat, 3 Sep 2016 08:31:39 -0400
Date: Sat, 3 Sep 2016 14:31:33 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Ingo Molnar <mingo@redhat.com>,
        Felipe Balbi <felipe.balbi@linux.intel.com>,
        USB list <linux-usb@vger.kernel.org>,
        Kernel development list <linux-kernel@vger.kernel.org>
Subject: Re: Memory barrier needed with wake_up_process()?
Message-ID: <20160903123133.GD2794@worktop>
References: <20160902184759.GB3663@linux.vnet.ibm.com>
 <Pine.LNX.4.44L0.1609021617520.2027-100000@iolanthe.rowland.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <Pine.LNX.4.44L0.1609021617520.2027-100000@iolanthe.rowland.org>
User-Agent: Mutt/1.5.22.1 (2013-10-16)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Fri, Sep 02, 2016 at 04:29:19PM -0400, Alan Stern wrote:
> I'm afraid so.  The code doesn't use wait_event(), in part because
> there's no wait_queue (since only one task is involved).

You can use wait_queue fine with just one task, and it would clean up
the code tremendously.

You can replace things like the earlier mentioned:

	while (bh->state != BUF_STATE_EMPTY) {
		rc = sleep_thread(common, false);
		if (rc)
			return rc;
	}

with:

	rc = wait_event_interruptible(&common->wq, bh->state == BUF_STATE_EMPTY);
	if (rc)
		return rc;

> But maybe there's another barrier which needs to be fixed.  Felipe, can
> you check to see if received_cbw() is getting called in
> get_next_command(), and if so, what value it returns?  Or is the
> preceding sleep_thread() the one that never wakes up?
> 
> It could be that the smp_wmb() in wakeup_thread() needs to be smp_mb().  
> The reason being that get_next_command() runs outside the protection of 
> the spinlock.

Being somewhat confused by the code, I fail to follow that argument.
wakeup_thread() is always called under that spinlock(), but since the
critical section is 2 stores, I fail to see how a smp_mb() can make any
difference over the smp_wmb() already there.