linux-kernel - Re: [RFC][PATCH 2/5] mips/atomic: Fix loongson_llsc

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Message-ID: <20190424125913.GO12232@hirez.programming.kicks-ass.net>
Date:   Wed, 24 Apr 2019 14:59:13 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     stern@...land.harvard.edu, akiyks@...il.com,
        andrea.parri@...rulasolutions.com, boqun.feng@...il.com,
        dlustig@...dia.com, dhowells@...hat.com, j.alglave@....ac.uk,
        luc.maranget@...ia.fr, npiggin@...il.com, paulmck@...ux.ibm.com,
        will.deacon@....com
Cc:     linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
        Huacai Chen <chenhc@...ote.com>,
        Huang Pei <huangpei@...ngson.cn>,
        Paul Burton <paul.burton@...s.com>
Subject: Re: [RFC][PATCH 2/5] mips/atomic: Fix loongson_llsc_mb() wreckage

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
mistakes.