[libcamera-devel] [PATCH 3/5] libcamera: thread: Fix race condition when dispatching messages

[Niklas Söderlund]

Hi Laurent,

Thanks for your patch.

On 2019-11-27 10:49:07 +0200, Laurent Pinchart wrote:
> The Object class stores the number of pending messages that have been
> posted for it, while the actual messages are stored in a per-thread list
> in the ThreadData class. When dispatching messages, the message is
> removed from the list, passed to the receiver, and the number of pending
> messages is updated.
> 
> In order to avoid too much contention on the list lock, the lock is
> dropped to pass the message to the receiver and then taken again. This
> creates a race condition window with Thread::removeMessages(), as the
> number of pending messages for a receiver is briefly out of sync with
> the pending messages list. The assertion at the end of removeMessages()
> thus sometimes fails.
> 
> Fix this by decrementing the pending messages counter before releasing
> the lock in Thread::dispatchMessages(). This fixes the slow message
> receiver test in MessageTest.
> 
> Fixes: 01b930964acd ("libcamera: thread: Add a messaging passing API")
> Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>

Reviewed-by: Niklas Söderlund <niklas.soderlund at ragnatech.se>

> ---
>  src/libcamera/thread.cpp | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/src/libcamera/thread.cpp b/src/libcamera/thread.cpp
> index 029a0e8fddd5..18ebd16a7e2f 100644
> --- a/src/libcamera/thread.cpp
> +++ b/src/libcamera/thread.cpp
> @@ -439,11 +439,11 @@ void Thread::dispatchMessages()
>  		Object *receiver = msg->receiver_;
>  		ASSERT(data_ == receiver->thread()->data_);
>  
> +		receiver->pendingMessages_--;
> +
>  		locker.unlock();
>  		receiver->message(msg.get());
>  		locker.lock();
> -
> -		receiver->pendingMessages_--;
>  	}
>  }
>  
> -- 
> Regards,
> 
> Laurent Pinchart
> 
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-- 
Regards,
Niklas Söderlund