[PATCH] D61115: Parallel: only allow the first TaskGroup to run tasks parallelly

[Fangrui Song via Phabricator via llvm-commits]

MaskRay added a comment.

A triply nested loop can easily reproduce the dead lock:

  #include <llvm/Support/Parallel.h>
  #include <unistd.h>
  
  int main() {
    int a[99] = {};
    llvm::parallel::for_each(llvm::parallel::par, a, a+99, [&](int x) {
      usleep(1000);
      llvm::parallel::for_each(llvm::parallel::par, a, a+99, [&](int x) {
        usleep(1000);
        llvm::parallel::for_each(llvm::parallel::par, a, a+99, [&](int x) {
          usleep(1000);
        });
      });
    });
  }

`strace -f ./a` => You soon see no `nanosleep` syscalls are made as all threads get stuck in `futex()`. This patch fixes that.


Repository:
  rL LLVM

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  https://reviews.llvm.org/D61115/new/

https://reviews.llvm.org/D61115