Messages in this thread | | | Date | Tue, 12 May 2020 22:14:56 +0100 | From | Will Deacon <> | Subject | Re: [PATCH v5 00/18] Rework READ_ONCE() to improve codegen |
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On Tue, May 12, 2020 at 09:07:55PM +0200, Peter Zijlstra wrote: > 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.
Hmm, I may have just run into this problem too. I'm using clang 11.0.1, but even if I do something like:
unsigned long __no_sanitize_or_inline foo(unsigned long *p) { return READ_ONCE_NOCHECK(*p); }
then I /still/ get calls to __tcsan_func_{entry,exit} emitted by the compiler. Marco -- how do you turn this thing off?!
I'm also not particularly fond of treating __{READ,WRITE}ONCE() as "atomic", since they're allowed to tear and I think callers should probably either be using data_race() explicitly or disabling instrumentation (assuming that's possible).
Will
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