On Tue, Jan 26, 2016 at 11:24:02AM +0100, Peter Zijlstra wrote:

On Thu, Jan 14, 2016 at 02:20:46PM -0800, Paul E. McKenney wrote:

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On Thu, Jan 14, 2016 at 01:24:34PM -0800, Leonid Yegoshin wrote:

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On 01/14/2016 12:48 PM, Paul E. McKenney wrote:

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So SYNC_RMB is intended to implement smp_rmb(), correct?

Yes.

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You could use SYNC_ACQUIRE() to implement read_barrier_depends() and
smp_read_barrier_depends(), but SYNC_RMB probably does not suffice.

If smp_read_barrier_depends() is used to separate not only two reads
but read pointer and WRITE basing on that pointer (example below) -
yes. I just doesn't see any example of this in famous
Documentation/memory-barriers.txt and had no chance to know what you
use it in this way too.

Well, Documentation/memory-barriers.txt was intended as a guide for Linux
kernel hackers, and not for hardware architects.

Yeah, this goes under the header: memory-barriers.txt is _NOT_ a
specification (I seem to keep repeating this).

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------------------------------------------------------------------------

commit 955720966e216b00613fcf60188d507c103f0e80
Author: Paul E. McKenney [off-list ref]
Date:   Thu Jan 14 14:17:04 2016 -0800

    documentation: Subsequent writes ordered by rcu_dereference()
    
    The current memory-barriers.txt does not address the possibility of
    a write to a dereferenced pointer.  This should be rare, 

How are these rare? Isn't:

	rcu_read_lock()
	obj = rcu_dereference(ptr);
	if (!atomic_inc_not_zero(&obj->ref))
		obj = NULL;
	rcu_read_unlock();

a _very_ common thing to do?

It is, but it provides its own barriers, so does not need to rely on
dependency ordering.

							Thanx, Paul