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    SubjectRe: [RFC][PATCH] mips: Fix arch_spin_unlock()
    Hi Paul,

    On Thu, Jan 28, 2016 at 02:31:31PM -0800, Paul E. McKenney wrote:
    > On Thu, Jan 28, 2016 at 09:57:19AM +0000, Will Deacon wrote:
    > > On Wed, Jan 27, 2016 at 03:38:36PM -0800, Paul E. McKenney wrote:
    > > > On Wed, Jan 27, 2016 at 03:21:58PM +0000, Will Deacon wrote:
    > > Yes, sorry for the shorthand:
    > >
    > > - Each paragraph is a separate thread
    > > - Wx=1 means WRITE_ONCE(x, 1), Rx=1 means READ_ONCE(x) returns 1
    > > - WxRel means smp_store_release(x,1), RxAcq=1 means smp_load_acquire(x)
    > > returns 1
    > > - Everything is initially zero
    > >
    > > > > and I suppose a variant of that:
    > > > >
    > > > >
    > > > > Wx=1
    > > > > WyRel=1
    > > > >
    > > > > RyAcq=1
    > > > > Wz=1
    > > > >
    > > > > Rz=1
    > > > > <address dependency>
    > > > > Rx=0
    > > >
    > > > Agreed, this would be needed as well, along with the read-read and
    > > > read-write variants. I picked the write-read version (Will's first
    > > > test above) because write-read reordering is the most likely on
    > > > hardware that I am aware of.
    > >
    > > Question: if you replaced "Wz=1" with "WzRel=1" in my second test, would
    > > it then be forbidden?
    >
    > On Power, yes. I would guess on ARM as well.

    Indeed.

    > For Linux in general, this is a question: How strict do we want to be
    > about matching the type of write with the corresponding read? My
    > default approach is to initially be quite strict and loosen as needed.
    > Here "quite strict" might mean requiring an rcu_assign_pointer() for
    > the write and rcu_dereference() for the read, as opposed to (say)
    > ACCESS_ONCE() for the read. (I am guessing that this would be too
    > tight, but it makes a good example.)
    >
    > Thoughts?

    That sounds broadly sensible to me and allows rcu_assign_pointer and
    rcu_dereference to be used as drop-in replacements for release/acquire
    where local transitivity isn't required. However, I don't think we can
    rule out READ_ONCE/WRITE_ONCE interactions as they seem to be used
    already in things like the osq_lock (albeit without the address
    dependency).

    Will

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