Re: [PATCH 1/3] linux: Move flockfile/_IO_flockfile into libc

[Florian Weimer <fweimer@redhat.com>]

* Adhemerval Zanella:

> diff --git a/stdio-common/flockfile.c b/stdio-common/flockfile.c
> index 0bc856a465..c838dcd538 100644
> --- a/stdio-common/flockfile.c
> +++ b/stdio-common/flockfile.c
> @@ -17,13 +17,18 @@
>     <https://www.gnu.org/licenses/>.  */
>  
>  #include <stdio.h>
> +#include <stdio-lock.h>
> +#include <sys/single_threaded.h>
>  
>  void
>  __flockfile (FILE *stream)
>  {
> +  bool multithread = __libc_single_threaded == 0;
> +  if (multithread)
> +    {
> +      stream->_flags2 |= _IO_FLAGS2_NEED_LOCK;
> +      _IO_lock_lock (*stream->_lock);
> +    }
>  }

I'm afraid this is not correct because a thread can become multithreaded
between the flockfile/funlockfile calls.  As a result, the lock state
would be wrong at funlockfile time.

In order to optimize locks in such cases, it is necessary to perform the
same actions as the locking/unlocking operations, but without atomic
instructions.

The stream->_flags2 update has a data race with other _flags2 updates
(e.g., in __malloc_stats), but that's a preexisting bug that should be
fixed separately (there might be other affected places).

Thanks,
Florian