Re: Optimization of conditional access to globals: thread-unsafe?

[Tomash Brechko <tomash dot brechko at gmail dot com>]

On Sun, Oct 21, 2007 at 17:26:02 +0200, Erik Trulsson wrote:
> Note that C99 is firmly based on a single-threaded execution model and says
> nothing whatsoever about what should happen or not happen in a threaded
> environment.  According to C99 a C compiler is allowed to generate such code
> as gcc does.

Yes, I understand that C99 doesn't concern threads per see, but I
wouldn't call it pro-single-threaded, rather thread-neutral.  I.e. the
standard isn't made explicitly incompatible with threads, it is
simply "says nothing about threads".


> If you are using some threaded environment then you will have to read the
> relevant standard for that to find out if it imposes any additional
> restricitions on a C compiler beyond what the C standard does.

All we have is POSIX, and it imposes very little on compiler I guess.


> I suspect that most of them will not say one way or the other about what
> should happen in this case, which means that you will have to assume the
> worst case and protect all calls to f() regardless of the value of the
> argument.

Well, assuming the worst case won't always work, that's why I asked
about reasonable boundary.  Consider the following (putting
style/efficiency matters aside):

  #include <pthread.h>

  #define N 100

  /* mutex[i] corresponds to byte[i].  */
  pthread_mutex_t mutex[N];
  char byte[N];

  void
  f(int i)
  {
    pthread_mutex_lock(&mutex[i]);
    byte[i] = 1;
    pthread_mutex_unlock(&mutex[i]);
  }


Is this code thread-safe?  Because from some POV C99 doesn't forbid to
load and store the whole word when single byte[i] is accessed (given
that C99 is pro-single-threaded).

But if C99 is thread-neutral, then it's compiler's responsibility to
ensure the same result as some abstract machine (which may be
sequential).  In this case the compiler should access the single byte,
no more.


OK, I've got your point, but I'm not satisfied :).


-- 
   Tomash Brechko