Re: [PATCH] powerpc/npu-dma.c: Fix deadlock in mmio_invalidate

[Alistair Popple <alistair@popple.id.au>]

> > Shouldn't that be enforced with READ_ONCE() then?

Yep, I can add that.

> Good point, although I think the acquire_* function itself may be called
> from a higher layer with the mmap_sem always held. I wonder if we need
> barriers around get and put mmio_atsd_reg.

test_and_set_bit() should imply a memory barrier so I don't think we need one
there (and looking at the implementation there is one). clear_bit() might need
one though. For that I guess I could use clear_bit_unlock()? There is also a
matching test_and_set_bit_lock() so I will submit a v2 which uses those instead
given we are using these like a lock.

> > > I don't think we can as the invalidate_range() function is called under the ptl
> > > spin-lock and is not allowed to sleep (at least according to
> > > include/linux/mmu_notifier.h).
> 
> I double checked, It's the reverse
> 
> 	 /*
>           * If both of these callbacks cannot block, mmu_notifier_ops.flags
>           * should have MMU_INVALIDATE_DOES_NOT_BLOCK set.
>           */

Argh, that must have been merged during the current window. Thanks for pointing
out - I will submit a seperate patch to update the mmu_notifier_ops.flags to set
MMU_INVALIDATE_DOES_NOT_BLOCK.

- Alistair

>          void (*invalidate_range_start)(struct mmu_notifier *mn,
>                                         struct mm_struct *mm,
>                                         unsigned long start, unsigned long end);
>          void (*invalidate_range_end)(struct mmu_notifier *mn,
>                                       struct mm_struct *mm,
>                                       unsigned long start, unsigned long end);
> > > 
> > > - Alistair
> > >   
> > > > Balbir
> > > >   
> > > 
> > > 
> > >   
> 
> I think it looks good to me otherwise,
> 
> Balbir Singh.
>