linux-kernel - Re: Memory barrier needed with wake_up

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Date:   Sat, 3 Sep 2016 10:49:39 -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, Alan Stern wrote:

> 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.

Actually, seeing it written out like this, one realizes that it really 
ought to be:

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

(Bear in mind also that on some platforms, the I/O operation is carried 
out by PIO rather than DMA.)

Also, the smp_wmb() in bulk_out_complete() looks unnecessary.  I can't 
remember why I put it there originally.

Alan Stern