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Re: Memory barrier needed with wake_up_process()?

From Peter Zijlstra <peterz@infradead.org>
Newsgroups linux.kernel
Subject Re: Memory barrier needed with wake_up_process()?
Date 2016-09-03 14:40 +0200
Message-ID <sdioN-58V-11@gated-at.bofh.it> (permalink)
References <sd1Hj-36f-23@gated-at.bofh.it> <sd3g6-4bf-3@gated-at.bofh.it>
Organization linux.* mail to news gateway

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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.

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Thread

Memory barrier needed with wake_up_process()? Alan Stern <stern@rowland.harvard.edu> - 2016-09-02 20:20 +0200
  Re: Memory barrier needed with wake_up_process()? "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-09-02 20:50 +0200
    Re: Memory barrier needed with wake_up_process()? Alan Stern <stern@rowland.harvard.edu> - 2016-09-02 22:30 +0200
      Re: Memory barrier needed with wake_up_process()? "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-09-03 11:10 +0200
      Re: Memory barrier needed with wake_up_process()? Peter Zijlstra <peterz@infradead.org> - 2016-09-03 14:40 +0200
        Re: Memory barrier needed with wake_up_process()? Alan Stern <stern@rowland.harvard.edu> - 2016-09-03 16:30 +0200
          Re: Memory barrier needed with wake_up_process()? Alan Stern <stern@rowland.harvard.edu> - 2016-09-03 16:50 +0200
            Re: Memory barrier needed with wake_up_process()? Peter Zijlstra <peterz@infradead.org> - 2016-09-05 10:40 +0200
              Re: Memory barrier needed with wake_up_process()? Alan Stern <stern@rowland.harvard.edu> - 2016-09-05 17:30 +0200
                Re: Memory barrier needed with wake_up_process()? Peter Zijlstra <peterz@infradead.org> - 2016-09-06 13:40 +0200
                Re: Memory barrier needed with wake_up_process()? Peter Zijlstra <peterz@infradead.org> - 2016-09-06 13:50 +0200
                Re: Memory barrier needed with wake_up_process()? Peter Zijlstra <peterz@infradead.org> - 2016-09-06 14:30 +0200
                Re: Memory barrier needed with wake_up_process()? Alan Stern <stern@rowland.harvard.edu> - 2016-09-06 16:50 +0200
                Re: Memory barrier needed with wake_up_process()? Peter Zijlstra <peterz@infradead.org> - 2016-09-06 17:10 +0200
                Re: Memory barrier needed with wake_up_process()? Felipe Balbi <felipe.balbi@linux.intel.com> - 2016-09-07 12:20 +0200
                Re: Memory barrier needed with wake_up_process()? Felipe Balbi <felipe.balbi@linux.intel.com> - 2016-09-06 13:50 +0200
  Re: Memory barrier needed with wake_up_process()? Peter Zijlstra <peterz@infradead.org> - 2016-09-02 21:30 +0200
    Re: Memory barrier needed with wake_up_process()? Alan Stern <stern@rowland.harvard.edu> - 2016-09-02 22:20 +0200
      Re: Memory barrier needed with wake_up_process()? Peter Zijlstra <peterz@infradead.org> - 2016-09-03 00:20 +0200
        Re: Memory barrier needed with wake_up_process()? Will Deacon <will.deacon@arm.com> - 2016-09-05 11:50 +0200
          Re: Memory barrier needed with wake_up_process()? Peter Zijlstra <peterz@infradead.org> - 2016-09-06 13:20 +0200
      Re: Memory barrier needed with wake_up_process()? Peter Zijlstra <peterz@infradead.org> - 2016-09-03 00:20 +0200
        Re: Memory barrier needed with wake_up_process()? Felipe Balbi <felipe.balbi@linux.intel.com> - 2016-09-03 09:00 +0200
          Re: Memory barrier needed with wake_up_process()? Peter Zijlstra <peterz@infradead.org> - 2016-09-03 14:50 +0200
            Re: Memory barrier needed with wake_up_process()? Felipe Balbi <felipe.balbi@linux.intel.com> - 2016-09-03 16:00 +0200
              Re: Memory barrier needed with wake_up_process()? Peter Zijlstra <peterz@infradead.org> - 2016-09-05 10:10 +0200
            Re: Memory barrier needed with wake_up_process()? Alan Stern <stern@rowland.harvard.edu> - 2016-09-03 16:20 +0200
              Re: Memory barrier needed with wake_up_process()? Peter Zijlstra <peterz@infradead.org> - 2016-09-05 10:10 +0200
                Re: Memory barrier needed with wake_up_process()? Alan Stern <stern@rowland.harvard.edu> - 2016-09-05 16:40 +0200
      Re: Memory barrier needed with wake_up_process()? Peter Zijlstra <peterz@infradead.org> - 2016-09-03 00:20 +0200

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