Re: [Bug #12574] possible circular locking dependency detected

[Thomas Hellström]

Eric Anholt wrote:
> On Wed, 2009-02-11 at 00:37 +0200, Michael S. Tsirkin wrote:
>   
> > Dave, dri guys,
> > Could you take a look at this circular dependency please (below)?  I
> > observe it when suspending laptop with radeon drm loaded and with
> > lockdep enabled. It seems that the root of the problem is that
> > various vm ops such as drm_vm_open, drm_mmap) are called with mm
> > semaphore taken, and take dev->struct_mutex. On the other hand,
> > drm_rmmap_locked is called with dev->struct_mutex, and calls mtrr_del
> > which depends on mm semaphore indirectly.
> >
> > What do you think?
> >     
>
> Yes, there are real lock inversions now due to the GTT mmap code.  It's
> going to be a pain to fix (I tried getting the mmap_sem -> struct_mutex
> path to go away, but the fact that mmap_sem is held over the fault
> handler pretty much kills that).  It's high on the list, though.
>   
Shouldn't it be possible to relax this given that in the fault() 
handler, the mmap_sem() is taken in read mode only. This means that it 
should be deadlock-safe (but still a little ugly) to take the mmap_sem() 
in read mode when the struct mutex is held.

Another quick way to fix potential deadlocks, would be to take the 
struct mutex in the following way in fault():

if (!mutex_trylock(&dev->struct_mutex)) {
     needs_resched();
     return VM_FAULT_NOPAGE;
}

/Thomas

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