Messages in this thread | | | From | Marco Elver <> | Date | Wed, 13 May 2020 00:00:55 +0200 | Subject | Re: [PATCH v5 00/18] Rework READ_ONCE() to improve codegen |
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On Tue, 12 May 2020 at 23:15, Will Deacon <will@kernel.org> wrote: > > 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?!
For Clang we have an option ("-mllvm -tsan-instrument-func-entry-exit=0"), for GCC, I don't think we have the option.
I had hoped we could keep these compiler changes optional for now, to not require a very recent compiler. I'll send a patch to enable the option, but keep it optional for now. Or do you think we require the compiler to support this? Because then we'll only support Clang.
> 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).
That point is fair enough. But how do we fix arch_atomic_{read,set} then?
Thanks, -- Marco
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