linux-kernel - Re: [PATCH 7/7] microblaze: Do atomic operations by using exclusive ops

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Message-ID: <20200213091651.GA14946@hirez.programming.kicks-ass.net>
Date:   Thu, 13 Feb 2020 10:16:51 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Michal Simek <michal.simek@...inx.com>
Cc:     linux-kernel@...r.kernel.org, monstr@...str.eu, git@...inx.com,
        arnd@...db.de,
        Stefan Asserhall load and store 
        <stefan.asserhall@...inx.com>, Boqun Feng <boqun.feng@...il.com>,
        Will Deacon <will@...nel.org>, paulmck@...nel.org
Subject: Re: [PATCH 7/7] microblaze: Do atomic operations by using exclusive
 ops

On Thu, Feb 13, 2020 at 09:58:49AM +0100, Peter Zijlstra wrote:

> The thing is, your bog standard LL/SC _SHOULD_ fail the SC if someone
> else does a regular store to the same variable. See the example in
> Documentation/atomic_t.txt.
> 
> That is, a competing SW/SWI should result in the interconnect responding
> with something other than EXOKAY, the SWX should fail and MSR[C] <- 1.

The thing is; we have code that relies on this behaviour. There are a
few crusty SMP archs that sorta-kinda limp along (mostly by disabling
some of the code and praying the rest doesn't trigger too often), but we
really should not allow more broken SMP archs.