Re: [PATCH -v2 4/6] locking, arch: Update spin_unlock_wait()

From: Peter Zijlstra
Date: Mon May 30 2016 - 05:26:20 EST

On Fri, May 27, 2016 at 03:34:13PM -0400, Chris Metcalf wrote:

> >Does TILE never speculate reads? Because in that case the control
> >dependency already provides a full load->load,store barrier and you'd
> >want smp_acquire__after_ctrl_dep() to be a barrier() instead of
> >smp_rmb().
> Yes, that's a good point. I didn't look at the definition of smp_acquire__after_ctrl_dep(),
> but it certainly sounds like that's exactly a compiler barrier for tile. There is no load
> speculation performed. The only out-of-order stuff that happens is in the memory
> subsystem: stores will become visible in arbitrary order, and loads will arrive in
> arbitrary order, but as soon as the result of a load is used in any other kind of
> instruction, the instruction issue will halt until the pending load(s) for the instruction
> operands are available.

OK; for now I'll just put in barrier() and a big comment, I'll look at
making smp_acquire__after_ctrl_dep() a proper (per arch) barrier later.
There's a little header head-ache involved.