Messages in this thread | | | From | Marco Elver <> | Date | Tue, 12 May 2020 19:53:00 +0200 | Subject | Re: [PATCH v5 00/18] Rework READ_ONCE() to improve codegen |
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On Tue, 12 May 2020 at 10:18, Peter Zijlstra <peterz@infradead.org> 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/20200110165636.28035-1-will@kernel.org > > v2: https://lore.kernel.org/lkml/20200123153341.19947-1-will@kernel.org > > v3: https://lore.kernel.org/lkml/20200415165218.20251-1-will@kernel.org > > v4: https://lore.kernel.org/lkml/20200421151537.19241-1-will@kernel.org > > > > 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) <peterz@infradead.org>
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().
Thanks, -- Marco
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