Re: Possible fix for bug #13165

[Torvald Riegel <triegel at redhat dot com>]

On Mon, 2014-08-18 at 17:27 -0400, Rich Felker wrote:
> On Mon, Aug 18, 2014 at 05:51:53PM -0300, Adhemerval Zanella wrote:
> > On 18-08-2014 16:53, Rich Felker wrote:
> > > A couple days ago I posted ideas for a fix for this issue on the bug
> > > tracker:
> > >
> > > https://sourceware.org/bugzilla/show_bug.cgi?id=13165#c38
> > >
> > > Anybody who does glibc development/builds/testing up for trying my
> > > idea and seeing if it works?
> > >
> > > Rich
> > >
> > If I understood correctly, you are proposing something like:
> > 
> > diff --git a/nptl/pthread_cond_wait.c b/nptl/pthread_cond_wait.c
> > index fc5eac4..a16c5d5 100644
> > --- a/nptl/pthread_cond_wait.c
> > +++ b/nptl/pthread_cond_wait.c
> > @@ -118,14 +118,6 @@ __pthread_cond_wait (cond, mutex)
> >    /* Make sure we are alone.  */
> >    lll_lock (cond->__data.__lock, pshared);
> >  
> > -  /* Now we can release the mutex.  */
> > -  err = __pthread_mutex_unlock_usercnt (mutex, 0);
> > -  if (__glibc_unlikely (err))
> > -    {
> > -      lll_unlock (cond->__data.__lock, pshared);
> > -      return err;
> > -    }
> > -
> >    /* We have one new user of the condvar.  */
> >    ++cond->__data.__total_seq;
> >    ++cond->__data.__futex;
> > @@ -153,6 +145,14 @@ __pthread_cond_wait (cond, mutex)
> >    /* Remember the broadcast counter.  */
> >    cbuffer.bc_seq = cond->__data.__broadcast_seq;
> >  
> > +  /* Now we can release the mutex.  */
> > +  err = __pthread_mutex_unlock_usercnt (mutex, 0);
> > +  if (__glibc_unlikely (err))
> > +    {
> > +      lll_unlock (cond->__data.__lock, pshared);
> > +      return err;
> > +    }
> > +
> >    do
> >      {
> >        unsigned int futex_val = cond->__data.__futex;
> > 
> > correct?
> 
> Yes, that looks like what I had in mind. But on second thought I'm not
> sure it should be necessary; I missed the internal lock that's being
> taken before the mutex is unlocked.
> 
> > I saw not NPTL issues in nether powerpc64 or x86_64.
> 
> You mean you can't reproduce the issue on these targets? Maybe it's
> only present on targets that are using non-default versions of the
> code. If so, and if the offending target-specific versions have been
> removed, maybe the bug is fixed now? I haven't had a chance to try the
> test case on latest glibc, but the bug was present on x86 (32-bit)
> last time I checked.
> 
> It would be nice if this bug has already been fixed without taking any
> specific action to do so...

The bug is not fixed.  It is an algorithmic issue -- though of course it
may or may not trigger in a particular execution depending on the
respective interleavings.