[RFC] dma-buf/fence: avoid holding lock while calling cb

[Rob Clark]

On Mon, Oct 17, 2016 at 4:25 AM, Maarten Lankhorst
<maarten.lankhorst at linux.intel.com> wrote:
> Op 16-10-16 om 18:03 schreef Rob Clark:
>> Currently with fence-array, we have a potential deadlock situation.  If we
>> fence_add_callback() on an array-fence, the array-fence's lock is aquired
>> first, and in it's ->enable_signaling() callback, it will install cb's on
>> it's array-member fences, so the array-member's lock is acquired second.
>>
>> But in the signal path, the array-member's lock is acquired first, and the
>> array-fence's lock acquired second.
>>
>> One approach to deal with this is avoid holding the fence's lock when
>> calling the cb.  It is a bit intrusive and I haven't fixed up all the
>> other drivers that call directly or indirectly fence_signal_locked().
>>
>> I guess the other option would be introduce a work-queue for array-fence?
>> Or??
> Enable signaling when creating the fence array is an option. As an optimization we don't enable
> signaling when creating a single fence, but when you merge fences you're probably interested
> in the result anyway.

I think what you mean is to fence_add_callback() on all the member
fences up-front, rather from ops->enable_signaling()?  I guess that
should work.

BR,
-R