Messages in this thread | | | Date | Tue, 12 May 2020 21:07:55 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v5 00/18] Rework READ_ONCE() to improve codegen |
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On Tue, May 12, 2020 at 07:53:00PM +0200, Marco Elver wrote: > 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().
But we can't... because I need arch_atomic_*() and __READ_ONCE() to not call out to _ANYTHING_.
Sadly, because the compilers are 'broken' that whole __no_sanitize thing didn't work, but I'll be moving a whole bunch of code into .c files with all the sanitizers killed dead. And we'll be validating it'll not be calling out to anything.
data_race() will include active calls to kcsan_{dis,en}able_current(), and this must not happen.
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