linux-kernel - Re: Memory barrier needed with wake_up

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Date:   Sat, 3 Sep 2016 10:26:58 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Peter Zijlstra <peterz@...radead.org>
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 Sat, 3 Sep 2016, Peter Zijlstra wrote:

> 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;

If someone wants to devote time and effort to cleaning up the driver, 
that would be a good start.

> > 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.

But sleep_thread() and the code that follows it are _not_ called under
the spinlock.  And the following code examines values that were written
by DMA, not by the CPU calling wakeup_thread().  (Although that CPU
_is_ the one that receives the DMA-completion notice.)

In other words, we have:

	CPU 0				CPU 1
	-----				-----
	Start DMA			Handle DMA-complete irq
	Sleep until bh->state		Set bh->state
					smp_wmb()
					Wake up CPU 0
	smp_rmb()
	Compute rc based on contents
		of the DMA buffer

This was written many years ago, at a time when I did not fully
understand all the details of memory ordering.  Do you agree that both
of those barriers should really be smp_mb()?  That's what Felipe has
been testing.

Alan Stern