Re: [PATCH 01/22] dma-fence: Serialise signal enabling (dma_fence_enable_sw_signaling)

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 03/10/2019 14:09, Chris Wilson wrote:
> Quoting Chris Wilson (2019-10-03 10:36:18)
>> Make dma_fence_enable_sw_signaling() behave like its
>> dma_fence_add_callback() and dma_fence_default_wait() counterparts and
>> perform the test to enable signaling under the fence->lock, along with
>> the action to do so. This ensure that should an implementation be trying
>> to flush the cb_list (by signaling) on retirement before freeing the
>> fence, it can do so in a race-free manner.
>>
>> See also 0fc89b6802ba ("dma-fence: Simply wrap dma_fence_signal_locked
>> with dma_fence_signal").
>>
>> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
>> ---
>>   drivers/dma-buf/dma-fence.c | 11 +++++------
>>   1 file changed, 5 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
>> index 2c136aee3e79..587727089134 100644
>> --- a/drivers/dma-buf/dma-fence.c
>> +++ b/drivers/dma-buf/dma-fence.c
>> @@ -285,19 +285,18 @@ void dma_fence_enable_sw_signaling(struct dma_fence *fence)
>>   {
>>          unsigned long flags;
>>   
>> +       if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags))
>> +               return;
>> +
>> +       spin_lock_irqsave(fence->lock, flags);
>>          if (!test_and_set_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT,
>>                                &fence->flags) &&
>> -           !test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags) &&
> 
> Staring at this in discussion with Tvrtko, we can't drop this check
> either. After which this looks almost identical with the other
> enable_signaling callers.

Ah yes true, DMA_FENCE_FLAG_SIGNALED_BIT has to be reliably checked 
before the enable_signalling callback. I retract my previous r-b. :)

Regards,

Tvrtko


>>              fence->ops->enable_signaling) {
>>                  trace_dma_fence_enable_signal(fence);
>> -
>> -               spin_lock_irqsave(fence->lock, flags);
>> -
>>                  if (!fence->ops->enable_signaling(fence))
>>                          dma_fence_signal_locked(fence);
>> -
>> -               spin_unlock_irqrestore(fence->lock, flags);
>>          }
>> +       spin_unlock_irqrestore(fence->lock, flags);
>>   }
>>   EXPORT_SYMBOL(dma_fence_enable_sw_signaling);
>>   
>> -- 
>> 2.23.0
>>
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