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

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Date:   Thu, 13 Feb 2020 10:04:03 +0000
From:   Will Deacon <will@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Michal Simek <michal.simek@...inx.com>,
        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>,
        paulmck@...nel.org
Subject: Re: [PATCH 7/7] microblaze: Do atomic operations by using exclusive
 ops

On Thu, Feb 13, 2020 at 10:16:51AM +0100, Peter Zijlstra wrote:
> 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.

I did find this in the linked pdf:

  | If the store [swx] is successful, the sequence of instructions from
  | the semaphore load to the semaphore store appear to be executed
  | atomically - no other device modified the semaphore location between
  | the read and the update.

which sounds like we're ok, although it could be better worded.

One part I haven't figured out is what happens if you take an interrupt
between the lwx and the swx and whether you can end up succeeding thanks
to somebody else's reservation. Also, the manual is silent about the
interaction with TLB invalidation and just refers to "address" when
talking about the reservation. What happens if a user thread triggers
CoW while another is in the middle of a lwx/swx?

Will