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    SubjectRe: [RFC PATCH 0/8] Rework READ_ONCE() to improve codegen
    On Fri, Jan 10, 2020 at 5:56 PM Will Deacon <will@kernel.org> wrote:
    >
    > Hi all,
    >
    > This is a follow-up RFC to the discussions we had on the mailing list at
    > the end of last year:
    >
    > https://lore.kernel.org/lkml/875zimp0ay.fsf@mpe.ellerman.id.au
    >
    > Unfortunately, we didn't get a "silver bullet" solution out of that
    > long thread, but I've tried to piece together some of the bits and
    > pieces we discussed and I've ended up with this series, which does at
    > least solve the pressing problem with the bitops for arm64.
    >
    > The rough summary of the series is:
    >
    > * Drop the GCC 4.8 workarounds, so that READ_ONCE() is a
    > straightforward dereference of a cast-to-volatile pointer.
    >
    > * Require that the access is either 1, 2, 4 or 8 bytes in size
    > (even 32-bit architectures tend to use 8-byte accesses here).
    >
    > * Introduce __READ_ONCE() for tearing operations with no size
    > restriction.
    >
    > * Drop pointer qualifiers from scalar types, so that volatile scalars
    > don't generate horrible stack-spilling mess. This is pretty ugly,
    > but it's also mechanical and wrapped up in a macro.
    >
    > * Convert acquire/release accessors to perform the same qualifier
    > stripping.
    >
    > I gave up trying to prevent READ_ONCE() on aggregates because it is
    > pervasive, particularly within the mm/ layer on things like pmd_t.
    > Thankfully, these don't tend to be volatile.
    >
    > I have more patches in this area because I'm trying to move all the
    > read_barrier_depends() magic into arch/alpha/, but I'm holding off until
    > we agree on this part first.

    Looks very nice overall, thanks for working on this.

    I've added a the series into my randconfig build setup to see
    if I run into build-time regressions. Unfortunately there are some
    conflicts with the kcsan patches in linux-next that I have to work
    around first.

    Arnd

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