Re: [RFC][PATCH 2/5] mips/atomic: Fix loongson_llsc_mb() wreckage

From: Peter Zijlstra
Date: Wed Apr 24 2019 - 08:59:41 EST

On Wed, Apr 24, 2019 at 02:36:58PM +0200, Peter Zijlstra wrote:
> The comment describing the loongson_llsc_mb() reorder case doesn't
> make any sense what so ever. Instruction re-ordering is not an SMP
> artifact, but rather a CPU local phenomenon. This means that _every_
> LL/SC loop needs this barrier right in front to avoid the CPU from
> leaking a memop inside it.
> For the branch speculation case; if futex_atomic_cmpxchg_inatomic()
> needs one at the bne branch target, then surely the normal
> __cmpxch_asmg() implementation does too. We cannot rely on the
> barriers from cmpxchg() because cmpxchg_local() is implemented with
> the same macro, and branch prediction and speculation are, too, CPU
> local.

Also; just doing them all makes for much simpler rules and less