[llvm-dev] Meet at MTV to discuss TSAN/android/aarch64?

[Renato Golin via llvm-dev]

Hi Jason,

I have a few additional points:

1. It works on glibc but not in bionic because glibc calls internal
allocation functions while bionic calls malloc/free, which get
instrumented when the thread is being destroyed. Calling bionic_malloc
on those cases should be enough to remove the problem altogether, but
we should not call bionic_malloc everywhere, since we want all other
calls to be instrumented and we don't want user facing behaviour to
change.

2. The changes to Android should be made in the implementation, not in
the header files. We're trying to avoid include nightmares and
pre-processor race conditions, as well as protecting users from
implementation details.

3. Google will prioritise pthread_barrier and other functionality that
helps TSAN works with bionic without the need for the TLS workaround
in compiler-rt. They'll also set up some internal bots to make sure we
test those things from now on.

cheers,
--renato

On 24 September 2015 at 11:18, Jason Kim <jasonk at codeaurora.org> wrote:
> Hi everyone.
>
> Thanks to everyone for taking the time to meet yesterday. I think it was
> very productive!
>
> I am writing to keep a "minutes" of the meet, as I remember them.
>
> Action Items:
>
> Jasonk:  split up bionic patch to explicitly replace CPP macros with actual
> calls, one file at a time.
> Google/Android: pthread_barrier interface in bionic
> Enh:  1 paragraph to explain "namespacing" in bionic?? (in case we need to
> reconsider macro replacement again?)
>
> Some technical notes/explanations that possibly weren't made clear at the
> meeting.
>
> The current compller-rt patch (on phabricator)  is already a pretty minimal
> set. The only real bit of actual functionality (other than the
> infrastructure bits to enable TSAN on android/aarch64) is the TLS
> workaround.
>
> i.e. the major pieces for my patch to compiler-rt are:
>
> infrastructure (new definitions, cmake changes etc..)
> TLS workaround
> pthread_barrier-like interface for tests
> disabling of some tests on android-aarch64
>
> My current hypothesis is that the 120 tests (out of 200 or so)  that are
> failing are due the behavioral mismatch of my "quick hack" to enable
> barrier-like behavior on android. Assuming that Google's implementation of
> the barrier interface on android/aarch64 is a better fit with linux-x86_64,
> this will hopefully result in more tests passing.
>
> Very important info regarding why __bionic_XXX() replacement for calls to
> XXX() will most likely need to be "global" (within bionic) in most cases:
>
> deadlocks, crashes in TSAN were all due to unexpected interceptions WHILE a
> call was already being intercepted by TSAN.
> The most obvious TSAN-only workaround is to ignore interceptions when this
> is taking place. This works for two threads, but   does not scale to more
> threads without the danger of TSAN missing events.
> Any unexpected nested call chains in bionic that TSAN currently intercepts
> will either need custom coding in TSAN to explicitly handle (for all
> possible cases in which such call chains can occur), OR replace those calls
> in bionic with non-interceptible ones. The former will obviously add much
> more complexity to TSAN (to wit, additional testing failure vulnerability),
> while the latter will mean more (mechanical) changes to BIONIC. Currently,
> my thinking is that the latter, (being mechanical in nature) is preferable.
> To ENH: The same holds for calls to tcgetattr() --> ioctl(), and isatty() ->
> tcgetattr() --> ioctl() call chains. TSAN intercepts  tcgetattr() and
> ioctl(), and is not expecting recursive interceptions during them. Its not
> at all clear how to handle this within TSAN, especially among multiple
> threads. Even if a logic can be worked out, it's fundamentally much clearer
> to simply disallow such interceptions to occur.
>
> Things that might require future discussion:
>
> need a build-bot/test bot on labs.llvm.org for android-bionic-aarch64-tsan
> tests to prevent insanity inducing regressions :-)
>
> Thanks for reading!
>
> -Jason
>
>
>
>