[PATCH v1] libcamera: v4l2_videodevice: `lastUsedCounter_` need not be atomic

[Barnabás Pőcze]

Hi


2025. 03. 19. 15:53 keltezéssel, Kieran Bingham írta:
> Quoting Barnabás Pőcze (2025-03-10 17:03:26)
>> The `V4L2BufferCache` type is not thread-safe. Its `lastUsedCounter_`
>> member is not used in contexts where its atomicity would matter.
>> So it does not need to be have an atomic type.
> 
> Any indications as to why this was using atomic ? Maybe just
> cautious development when it was first put in ?

Not sure. It was Laurent who suggested it: https://patchwork.libcamera.org/patch/2878/#3889

There is only a single place where `lastUsedCounter_` might be used
by multiple threads: `V4L2BufferCache::get()`. But that function
is not thread safe, so either `lastUsedCounter_` is unnecessarily
atomic or race conditions are rampant already. As far as I can
tell it is the former.


Regards,
Barnabás Pőcze


> 
> As long as we're convinced it's still safe and passes the tests, I'm
> fine with it. It's certainly simpler code.
> 
> Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
> 
>> Signed-off-by: Barnabás Pőcze <barnabas.pocze at ideasonboard.com>
>> ---
>>   include/libcamera/internal/v4l2_videodevice.h | 2 +-
>>   src/libcamera/v4l2_videodevice.cpp            | 4 ++--
>>   2 files changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/libcamera/internal/v4l2_videodevice.h b/include/libcamera/internal/v4l2_videodevice.h
>> index f021c2a01..e52b50c67 100644
>> --- a/include/libcamera/internal/v4l2_videodevice.h
>> +++ b/include/libcamera/internal/v4l2_videodevice.h
>> @@ -158,7 +158,7 @@ private:
>>                  std::vector<Plane> planes_;
>>          };
>>   
>> -       std::atomic<uint64_t> lastUsedCounter_;
>> +       uint64_t lastUsedCounter_;
>>          std::vector<Entry> cache_;
>>          /* \todo Expose the miss counter through an instrumentation API. */
>>          unsigned int missCounter_;
>> diff --git a/src/libcamera/v4l2_videodevice.cpp b/src/libcamera/v4l2_videodevice.cpp
>> index e241eb47b..f5b3fa09d 100644
>> --- a/src/libcamera/v4l2_videodevice.cpp
>> +++ b/src/libcamera/v4l2_videodevice.cpp
>> @@ -190,7 +190,7 @@ V4L2BufferCache::V4L2BufferCache(const std::vector<std::unique_ptr<FrameBuffer>>
>>   {
>>          for (const std::unique_ptr<FrameBuffer> &buffer : buffers)
>>                  cache_.emplace_back(true,
>> -                                   lastUsedCounter_.fetch_add(1, std::memory_order_acq_rel),
>> +                                   lastUsedCounter_++,
>>                                      *buffer.get());
>>   }
>>   
>> @@ -258,7 +258,7 @@ int V4L2BufferCache::get(const FrameBuffer &buffer)
>>                  return -ENOENT;
>>   
>>          cache_[use] = Entry(false,
>> -                           lastUsedCounter_.fetch_add(1, std::memory_order_acq_rel),
>> +                           lastUsedCounter_++,
>>                              buffer);
>>   
>>          return use;
>> -- 
>> 2.48.1
>>