On Tue, Feb 14, 2017 at 6:22 PM, Adar Dembo <a...@cloudera.com> wrote:
> Any idea why this only surfaced recently? There's a call to > std::call_once in util/logging.cc that's been there for almost a year. > I'm guessing that one is only very rarely called (on crash) and would only be called by multiple threads at or near the same time in an even rarer case (two threads crash simultaneously). > In any case, I'm not a huge fan of #2. #1 and #3 both require a patch, > but I think #1 is more likely to get noticed (and reverted) upon an > LLVM 4.0 upgrade. So I think I vote for #1, though #3 is reasonable > too. > OK, I'll go with #1. I tested that the patch applies cleanly and fixes a manufactured test case I wrote. > > On Tue, Feb 14, 2017 at 6:09 PM, Todd Lipcon <t...@cloudera.com> wrote: > > BTW, I should add that libstdcxx as included in gcc 4.9.2 (devtoolset on > > el6) doesn't have this bug, since it just wraps pthread_once. So, it's > > probably not a _real_ issue since we don't use libc++ for production > builds. > > > > -Todd > > > > On Tue, Feb 14, 2017 at 6:05 PM, Todd Lipcon <t...@cloudera.com> wrote: > > > >> I noticed that a lot of our tests have gotten flaky with a TSAN race > >> reported around SSL initialization. After digging into a bit, I found > that > >> our version of libcxx has a bug in std::once where it's using a relaxed > >> load instead of an acquire load to check the flag of whether it has run > yet. > >> > >> I went to go report this upstream and discovered it's actually already > >> fixed: > >> https://github.com/llvm-mirror/libcxx/commit/ > >> 4dbd4fcf8626453949625bf36976e668094cfbb1 > >> > >> I'd still like to resolve the race, though. A couple options: > >> 1) apply this patch to our libcxx > >> 2) use GoogleOnce instead of std::call_once > >> 3) suppress it until we upgrade to llvm 4.0 with the newer libcxx, which > >> should likely be released in a few weeks > >> > >> Any opinions? > >> -Todd > >> -- > >> Todd Lipcon > >> Software Engineer, Cloudera > >> > > > > > > > > -- > > Todd Lipcon > > Software Engineer, Cloudera > -- Todd Lipcon Software Engineer, Cloudera
