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    SubjectRe: [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:
    > > On Tue, Dec 05, 2017 at 12:08:01PM -0800, Paul E. McKenney wrote:
    > > > On Tue, Dec 05, 2017 at 09:51:48PM +0200, Michael S. Tsirkin wrote:
    > > > > On Tue, Dec 05, 2017 at 11:33:39AM -0800, Paul E. McKenney wrote:
    > > > > > On Tue, Dec 05, 2017 at 09:24:21PM +0200, Michael S. Tsirkin wrote:
    > > >
    > > > [ . . . ]
    > > >
    > > > > > > 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!

    --
    MST

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