Re: [PATCH] Avoid RMW of flags2 outside lock (BZ #27842)

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 19/05/2022 12:10, Wilco Dijkstra wrote:
> 
> Remove an unconditional RMW on flags2 in flockfile - if _IO_FLAGS2_NEED_LOCK
> is set, we are single-threaded and we can safely clear the flag. This fixes
> BZ #27842.
> 

I don't think this is correct because if the caller issues pthread_create 
after flockfile, funlockfile will not issues the correct operations.  I
have a fix that uses a different locking mechanism where the _IO_FLAGS2_NEED_LOCK
is removed by moving both the thread id and single-thread optimization to the
locks itself (on Linux tid has at maximum 30-bits, we can use 1 bits for the
single-thread optimization and 1 bits for congestion optimization).

The issue is now requires slight more memory operations to check if you
can use single-thread optimization, since the FILE lock is not always
present and you need to first check __flags.

I would say that with currency scheme where _IO_FLAGS2_NEED_LOCK is stick,
this is a benign data race (although still undesirable). 

> ---
> 
> diff --git a/stdio-common/flockfile.c b/stdio-common/flockfile.c
> index a5decb450f8d477e3105d02661282afeab58f88b..7ba9ab59082d2c1dfba7d5e9a91175c9dec7ec49 100644
> --- a/stdio-common/flockfile.c
> +++ b/stdio-common/flockfile.c
> @@ -22,7 +22,10 @@
>  void
>  __flockfile (FILE *stream)
>  {
> -  stream->_flags2 |= _IO_FLAGS2_NEED_LOCK;
> +  /* If we're single-threaded, turn off single-thread optimizations
> +     when locking a file.  Avoid updating flags2 if multi-threaded.  */
> +  if (!_IO_need_lock (stream))
> +    stream->_flags2 |= _IO_FLAGS2_NEED_LOCK;
>    _IO_lock_lock (*stream->_lock);
>  }
>  weak_alias (__flockfile, flockfile);