linux-kernel - Re: Memory barrier needed with wake_up

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Message-ID: <20160903123133.GD2794@worktop>
Date:   Sat, 3 Sep 2016 14:31:33 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...hat.com>,
        Felipe Balbi <felipe.balbi@...ux.intel.com>,
        USB list <linux-usb@...r.kernel.org>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Memory barrier needed with wake_up_process()?

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.