Re: [c++-pthreads] Re: thread-safety definition

[Fergus Henderson <fjh@xxxxxxxxxxx>]

On 12-Jan-2004, Ted Baker <baker@xxxxxxxxxx> wrote:
> > The problem is that cleanup -- e.g. execution of C++ destructors on
> > stack unwinding -- may involve calls to I/O routines that are themselves
> > cancellation points.  If these routines fail, then the cleanup functions
> > will not achieve their intended purpose; they will not be able to release
> > the resources that the thread holds.  Nathan's model implies that they
> > will fail.
> 
> This cannot happen if cancellation is automagically disabled during
> execution of (all) destructors, along with the stack unwinding code
> that is performed during exception processing.

If all cleanup is done via destructors (or pthread_cleanup_push,
which can be handled similarly), you are right.

But recall that in Nathan's model, Posix functions that return status
values will indicate cancellation by returning a failure status and
setting errno to ECANCELLED.  In this situation, cleanup actions will
often be performed directly, rather than via destructors.  Once some
code has called a cancellation point that fails with ECANCELLED, there is
no way for the implementation to tell whether any further code which is
executed is cleanup code or just a resumption of the thread's main loop.
For cleanup code, cancellation must be disabled, so that the cleanup
code can perform I/O to release resources.  But for any long-running
non-cleanup code, cancellation must be reenabled; otherwise, we would
be failing to provide the limited assurance that cancellation will not
be accidentally ignored, which stickiness of cancellation was intended
to ensure.

Since the implementation can't tell, the code must be modified to
explicitly enable and/or disable cancellation at appropriate points.

Nathan's model was intended to preserve existing code that propagated
error returns, and that did not expect functions like read() or printf()
to throw exceptions.  However, because of the problems described above,
such existing code would still need to be changed to make cleanup work
properly.  So I don't think Nathan's model achieves his intended aim.

-- 
Fergus Henderson <fjh@xxxxxxxxxxx>  |  "I have always known that the pursuit
The University of Melbourne         |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.