[PATCH 1/4] dma-buf: Check status of enable-signaling bit on debug
Tvrtko Ursulin
tvrtko.ursulin at linux.intel.com
Mon Sep 5 16:39:39 UTC 2022
On 05/09/2022 12:21, Christian König wrote:
> Am 05.09.22 um 12:56 schrieb Arvind Yadav:
>> The core DMA-buf framework needs to enable signaling
>> before the fence is signaled. The core DMA-buf framework
>> can forget to enable signaling before the fence is signaled.
>> To avoid this scenario on the debug kernel, check the
>> DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT status bit before checking
>> the signaling bit status to confirm that enable_signaling
>> is enabled.
>
> You might want to put this patch at the end of the series to avoid
> breaking the kernel in between.
>
>>
>> Signed-off-by: Arvind Yadav <Arvind.Yadav at amd.com>
>> ---
>> include/linux/dma-fence.h | 5 +++++
>> 1 file changed, 5 insertions(+)
>>
>> diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
>> index 775cdc0b4f24..60c0e935c0b5 100644
>> --- a/include/linux/dma-fence.h
>> +++ b/include/linux/dma-fence.h
>> @@ -428,6 +428,11 @@ dma_fence_is_signaled_locked(struct dma_fence
>> *fence)
>> static inline bool
>> dma_fence_is_signaled(struct dma_fence *fence)
>> {
>> +#ifdef CONFIG_DEBUG_FS
>
> CONFIG_DEBUG_FS is certainly wrong, probably better to check for
> CONFIG_DEBUG_WW_MUTEX_SLOWPATH here.
>
> Apart from that looks good to me,
What's the full story in this series - I'm afraid the cover letter does not make it clear to a casual reader like myself? Where does the difference between debug and non debug kernel come from?
And how do the proposed changes relate to the following kerneldoc excerpt:
* Since many implementations can call dma_fence_signal() even when before
* @enable_signaling has been called there's a race window, where the
* dma_fence_signal() might result in the final fence reference being
* released and its memory freed. To avoid this, implementations of this
* callback should grab their own reference using dma_fence_get(), to be
* released when the fence is signalled (through e.g. the interrupt
* handler).
*
* This callback is optional. If this callback is not present, then the
* driver must always have signaling enabled.
Is it now an error, or should be impossible condition, for "is signaled" to return true _unless_ signaling has been enabled?
If the statement (in a later patch) is signalling should always be explicitly enabled by the callers of dma_fence_add_callback, then what about the existing call to __dma_fence_enable_signaling from dma_fence_add_callback?
Or if the rules are changing shouldn't kerneldoc be updated as part of the series?
Regards,
Tvrtko
> Christian.
>
>> + if (!test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &fence->flags))
>> + return false;
>> +#endif
>> +
>> if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags))
>> return true;
>
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