linux-kernel - Re: [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops)

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Date:   Fri, 6 Dec 2019 14:16:50 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>, dja@...ens.net,
        elver@...gle.com, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, christophe.leroy@....fr,
        linux-s390@...r.kernel.org, linux-arch@...r.kernel.org,
        x86@...nel.org, kasan-dev@...glegroups.com,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag
 (topic/kasan-bitops)

On Fri, Dec 06, 2019 at 11:46:11PM +1100, Michael Ellerman wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA256
> 
> Hi Linus,
> 
> Please pull another powerpc update for 5.5.
> 
> As you'll see from the diffstat this is mostly not powerpc code. In order to do
> KASAN instrumentation of bitops we needed to juggle some of the generic bitops
> headers.
> 
> Because those changes potentially affect several architectures I wasn't
> confident putting them directly into my tree, so I've had them sitting in a
> topic branch. That branch (topic/kasan-bitops) has been in linux-next for a
> month, and I've not had any feedback that it's caused any problems.
> 
> So I think this is good to merge, but it's a standalone pull so if anyone does
> object it's not a problem.

No objections, but here:

  https://git.kernel.org/pub/scm/linux/kernel/git/powerpc/linux.git/commit/?h=topic/kasan-bitops&id=81d2c6f81996e01fbcd2b5aeefbb519e21c806e9

you write:

  "Currently bitops-instrumented.h assumes that the architecture provides
atomic, non-atomic and locking bitops (e.g. both set_bit and __set_bit).
This is true on x86 and s390, but is not always true: there is a
generic bitops/non-atomic.h header that provides generic non-atomic
operations, and also a generic bitops/lock.h for locking operations."

Is there any actual benefit for PPC to using their own atomic bitops
over bitops/lock.h ? I'm thinking that the generic code is fairly
optimal for most LL/SC architectures.

I've been meaning to audit the various architectures and move them over,
but alas, it's something I've not yet had time for...