Re: [PATCH] New condvar implementation that provides stronger ordering guarantees.

[Torvald Riegel <triegel at redhat dot com>]

On Mon, 2015-02-23 at 12:59 -0500, Rich Felker wrote:
> On Mon, Feb 23, 2015 at 12:26:49PM +0100, Torvald Riegel wrote:
> > On Sun, 2015-02-22 at 17:37 -0500, Rich Felker wrote:
> > > On Fri, Feb 20, 2015 at 07:18:27PM +0100, Torvald Riegel wrote:
> > > > +   Limitations:
> > > > +   * This condvar isn't designed to allow for more than
> > > > +     WSEQ_THRESHOLD * (1 << (sizeof(GENERATION) * 8 - 1)) calls to
> > > > +     __pthread_cond_wait.  It probably only suffers from potential ABA issues
> > > > +     afterwards, but this hasn't been checked nor tested.
> > > > +   * More than (1 << (sizeof(QUIESCENCE_WAITERS) * 8) -1 concurrent waiters
> > > > +     are not supported.
> > > > +   * Beyond what is allowed as errors by POSIX or documented, we can also
> > > > +     return the following errors:
> > > > +     * EPERM if MUTEX is a recursive mutex and the caller doesn't own it.
> > > 
> > > This is not beyond POSIX; it's explicitly specified as a "shall fail".
> > 
> > http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_cond_wait.html
> > 
> > [EPERM]
> >         The mutex type is PTHREAD_MUTEX_ERRORCHECK or the mutex is a
> >         robust mutex, and the current thread does not own the mutex.
> > 
> > POSIX does not seem to allow EPERM for *recursive mutexes*.  Is there an
> > update that I'm missing?
> 
> Well it doesn't specifically require it for recursive (I missed that)
> but it also doesn't disallow it.

Yes, it doesn't disallow it explicitly, but for it to be allowed, it
would have to be listed at least in the "may fail", right?

> > > > +     * EOWNERDEAD or ENOTRECOVERABLE when using robust mutexes.  Unlike
> > > > +       for other errors, this can happen when we re-acquire the mutex; this
> > > > +       isn't allowed by POSIX (which requires all errors to virtually happen
> > > > +       before we release the mutex or change the condvar state), but there's
> > > > +       nothing we can do really.
> > > 
> > > Likewise these are "shall fail" errors specified by POSIX, and while
> > > it's not clearly written in the specification, it's clear that they
> > > only happen on re-locking.
> > 
> > Yes, they are "shall fail".  I also agree that POSIX *should* make it
> > clear that they can happen after releasing and when acquiring the mutex
> > again -- but that's not what the spec says:
> > 
> > "Except in the case of [ETIMEDOUT], all these error checks shall act as
> > if they were performed immediately at the beginning of processing for
> > the function and shall cause an error return, in effect, prior to
> > modifying the state of the mutex[...]"
> 
> OK, then I think that text is a bug. There's no way that mutex locking
> errors could meaningful before the mutex is unlocked.
> 
> > Until these two get clarified in the spec, I consider the comments
> > correct.  We can certainly extend them and document why we thing this
> > behavior is The Right Thing To Do.  But we need to document where we
> > deviate from what the spec states literally.
> 
> Yes. Would you like to submit the bug report or should I?

If you have some time, please do.