On Fri, Jan 10, 2020 at 5:56 PM Will Deacon <will@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx > > 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
