Re: Thread-, Signal- and Cancellation-safety documentation

[Torvald Riegel <triegel at redhat dot com>]

On Wed, 2013-06-05 at 13:21 -0400, Rich Felker wrote:
> On Wed, Jun 05, 2013 at 05:27:58PM +0200, Torvald Riegel wrote:
> > On Sun, 2013-06-02 at 02:06 -0300, Alexandre Oliva wrote:
> > > On Jun  1, 2013, Alexandre Oliva <aoliva@redhat.com> wrote:
> > > 
> > > > What must not happen is for mem synch primitives, called by users, to
> > > > fail to propagate effects of posix API calls executed locally to be
> > > > published to global memory, and make effects of API calls executed in
> > > > other threads before their latest mem synch primitives visible to the
> > > > local thread.  Other than that, all bets are off.
> > > 
> > > Now, bringing that back to the topic that matters to the project I'm on,
> > > and trying to show that the guarantees you're looking for are not
> > > given by posix, consider this:
> > > 
> > > char a, b; // globals
> > > int pipefd[2]; // the two ends of a pipe internal to this process
> > > 
> > > thread1 {
> > >   a = 42;
> > >   b = 6 * 9;
> > >   write (pipefd[1], &b, 1);
> > > }
> > > 
> > > thread2 {
> > >   read (pipefd[0], &a, 1);
> > >   printf ("%i\n", a);
> > > }
> > > 
> > > 
> > > Now, obviously there's a happens-before relationship between the write
> > > and the read, right?
> > 
> > More precisely, there is an ordering implied between the return of the
> > read and the start of the write.
> > This would make the program have a data race because read() will access
> > a prior to this ordering.
> > 
> > So let's use this example instead:
> > 
> > thread1 {
> >   a = 42;
> >   b = a;  // Reveal the program logic / intra-thread order to thread2
> >   write (pipefd[1], &b, 1);
> > }
> 
> This does not change anything. The compiler may freely reorder the
> writes to a and b.

The order of the writes to a and b is irrelevant (this piece of the
example was just change to highlight a potential program logic
relation).  The key point is whether write() synchronizes or not.  For
example, if it has a release fence or something similar (or if the
compiler can't prove that it doesn't), then both writes to a and b can't
be reordered to after the fence.