[llvm-dev] Race condition in raw_ostream

[Mehdi Amini via llvm-dev]

> On Dec 7, 2016, at 1:52 AM, Viacheslav Nikolaev via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> This code from raw_ostream.h is really racy:
> 
>   raw_ostream &operator<<(StringRef Str) {
>     // Inline fast path, particularly for strings with a known length.
>     size_t Size = Str.size();
> 
>     // Make sure we can use the fast path.
>     if (Size > (size_t)(OutBufEnd - OutBufCur))
>       return write(Str.data(), Size);
> 
>     if (Size) {
>       memcpy(OutBufCur, Str.data(), Size);
>       OutBufCur += Size;
>     }
>     return *this;
>   }

I don’t believe "the is racy” is an appropriate qualification, “buffered raw_ostream are not providing a thread-safe API" seems more accurate to me.


> Of course, one might wonder why someone would need to output to a stream from multiple threads at the same time.
> 
> But imagine someone might get logs to dbgs() or errs() running the backend for a target in multiple threads.

These are unbuffered, I wouldn’t expect a race in the code you list above. I believe it’ll always forward directly to raw_fd_ostream::write_impl(), which is calling the libc ::write().

— 
Mehdi