Re: Possible fix for bug #13165

[Rich Felker <dalias at libc dot org>]

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...

Rich