Re: [patch 0/3] futex/rtmutex: Fix issues exposed by trinity

[Peter Zijlstra]

On Wed, May 14, 2014 at 11:53:44AM +0200, Thomas Gleixner wrote:
> > What error would we return?
> >
> > This particular case is a serious error for which we have no good error code
> > to return to userspace. It's an implementation defect, a bug, we should probably
> > assert instead of pausing.
> 
> Errm.
> 
> http://pubs.opengroup.org/onlinepubs/7908799/xsh/pthread_mutex_lock.html
> 
>  The pthread_mutex_lock() function may fail if:
> 
>   [EDEADLK]
>     The current thread already owns the mutex. 
> 
> That's a exactly the error code, which the kernel returns when it
> detects a deadlock. 
> 
> And glibc returns EDEADLK at a lot of places already. So in that case
> it's not a serious error? Because it's detected by glibc. You can't be
> serious about that.
> 
> So why is a kernel detected deadlock different? Because it detects not
> only AA, it detects ABBA and more. But it's still a dead lock. And
> while posix spec only talks about AA, it's the very same issue.
> 
> So why not propagate this to the caller so he gets an alert right away
> instead of letting him attach a debugger, and scratch his head and
> lookup glibc source to find out why the hell glibc called pause.

  http://pubs.opengroup.org/onlinepubs/009695399/functions/pthread_mutex_lock.html

  The pthread_mutex_lock() function may fail if:

  [EDEADLK]
      A deadlock condition was detected or the current thread already owns the mutex.

Which is explicitly wider than the AA recursion and fully supports the
full lock graph traversal we do.

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