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

[Thomas Gleixner]

On Wed, 14 May 2014, Peter Zijlstra wrote:
> 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

Yuck. I should not have used the first link Gurgle brought up.
 
>   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.
 
Definitely. It's what the kernel does :)

Thanks,

	tglx
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