On Tue, 12 May 2020 at 19:53, Marco Elver <[email protected]> wrote: > > On Tue, 12 May 2020 at 10:18, Peter Zijlstra <[email protected]> wrote: > > > > On Mon, May 11, 2020 at 09:41:32PM +0100, Will Deacon wrote: > > > Hi folks, > > > > > > (trimmed CC list since v4 since this is largely just a rebase) > > > > > > This is version five of the READ_ONCE() codegen improvement series that > > > I've previously posted here: > > > > > > RFC: https://lore.kernel.org/lkml/[email protected] > > > v2: https://lore.kernel.org/lkml/[email protected] > > > v3: https://lore.kernel.org/lkml/[email protected] > > > v4: https://lore.kernel.org/lkml/[email protected] > > > > > > The main change since v4 is that this is now based on top of the KCSAN > > > changes queued in -tip (locking/kcsan) and therefore contains the patches > > > necessary to avoid breaking sparc32 as well as some cleanups to > > > consolidate {READ,WRITE}_ONCE() and data_race(). > > > > > > Other changes include: > > > > > > * Treat 'char' as distinct from 'signed char' and 'unsigned char' for > > > __builtin_types_compatible_p() > > > > > > * Add a compile-time assertion that the argument to READ_ONCE_NOCHECK() > > > points at something the same size as 'unsigned long' > > > > > > I'm happy for all of this to go via -tip, or I can take it via arm64. > > > > Looks good to me; Thanks! > > > > Acked-by: Peter Zijlstra (Intel) <[email protected]> > > I just ran a bunch of KCSAN tests. While this series alone would have > passed the tests, there appears to be a problem with > __READ_ONCE/__WRITE_ONCE. I think they should already be using > 'data_race()', as otherwise we will get lots of false positives in > future. > > I noticed this when testing -tip/locking/kcsan, which breaks > unfortunately, because I see a bunch of spurious data races with > arch_atomic_{read,set} because "locking/atomics: Flip fallbacks and > instrumentation" changed them to use __READ_ONCE()/__WRITE_ONCE(). > From what I see, the intent was to not double-instrument, > unfortunately they are still double-instrumented because > __READ_ONCE/__WRITE_ONCE doesn't hide the access from KCSAN (nor KASAN > actually). I don't think we can use __no_sanitize_or_inline for the > arch_ functions, because we really want them to be __always_inline > (also to avoid calls to these functions in uaccess regions, which > objtool would notice). > > I think the easiest way to resolve this is to wrap the accesses in > __*_ONCE with data_race().
I just sent https://lkml.kernel.org/r/[email protected] -- note that, using __*_ONCE in arch_atomic_{read,set} will once again double-instrument with this. Overall there are 2 options: 1. provide __READ_ONCE/__WRITE_ONCE wrapped purely in data_race(), or 2. make __READ_ONCE/__WRITE_ONCE perform an atomic check so we may still catch races with plain accesses. The patch I sent does (2). It is inevitable that these will be used in places that we did not expect, purely to get around the type check, which is why I thought it might be the more conservative approach. Thoughts? Thanks, -- Marco
