Re: [PATCH tip/core/rcu 21/21] drivers/vhost: Remove now-redundant read_barrier_depends()

On Tue, Dec 05, 2017 at 01:36:44PM -0800, Paul E. McKenney wrote:

On Tue, Dec 05, 2017 at 11:24:49PM +0200, Michael S. Tsirkin wrote:

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On Tue, Dec 05, 2017 at 12:08:01PM -0800, Paul E. McKenney wrote:

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On Tue, Dec 05, 2017 at 09:51:48PM +0200, Michael S. Tsirkin wrote:

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On Tue, Dec 05, 2017 at 11:33:39AM -0800, Paul E. McKenney wrote:

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On Tue, Dec 05, 2017 at 09:24:21PM +0200, Michael S. Tsirkin wrote:

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and this barrier is no longer paired with anything until
you realize there's a dependency barrier within READ_ONCE.

Barrier pairing was a useful tool to check code validity,
maybe there are other, better tools now.

There are quite a few people who say that smp_store_release() is
easier for the tools to analyze than is smp_wmb().  My experience with
smp_read_barrier_depends() and rcu_dereference() leads me to believe
that they are correct.

OK, but smp_store_release is still not paired with anything since we
rely on READ_ONCE to include the implicit dpendendency barrier.

Why wouldn't you consider the smp_store_release() to be paired with
the new improved READ_ONCE()?

READ_ONCE is really all over the place (some code literally replaced all
memory accesses with READ/WRITE ONCE).

And I also prefer smp_wmb as it seems to be cheaper on ARM.

Would an API like WRITE_POINTER()/smp_store_pointer make sense,
and READ_POINTER for symmetry?

What we do in some code is to comment the pairings, allowing the other
side of the pairing to be easily located.  Would that work for you?

							Thanx, Paul

Yes, that's exactly what I did for now.

Thanks!

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MST