[PATCH v2 3/6] test: fence: Fix race condition

[Kieran Bingham]

Quoting Laurent Pinchart (2024-05-29 16:43:38)
> The fence test is racy, as it relies on the main loop being executed
> between completion of request signalledRequestId_ and
> signalledRequestId_ + 1. This usually happens, but is not guaranteed.
> 
> To fix the race condition, change the request identification logic by
> replacing usage of the cookie value, which is zero-based and wraps
> around at nbuffers_ - 1, with a completed request counter that is
> one-based and doesn't wrap. The completedRequestId_, expiredRequestId_
> and signalledRequestId_ variables now track the identifier of the last
> request that has completed, the request whose fence will time out, and
> the request whose fence will be signalled.
> 
> Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> ---
> Changes since v1:
> 
> - Add and update comments
> - Fix typo in commit message
> ---
>  test/fence.cpp | 57 +++++++++++++++++++++++++++++++-------------------
>  1 file changed, 36 insertions(+), 21 deletions(-)
> 
> diff --git a/test/fence.cpp b/test/fence.cpp
> index c282886287aa..a8fba7284d82 100644
> --- a/test/fence.cpp
> +++ b/test/fence.cpp
> @@ -57,8 +57,11 @@ private:
>         bool expectedCompletionResult_ = true;
>         bool setFence_ = true;
>  
> -       unsigned int completedRequest_;
> -
> +       /*
> +        * Request IDs track the number of requests that have completed. They
> +        * are one-based, and don't wrap.
> +        */
> +       unsigned int completedRequestId_;
>         unsigned int signalledRequestId_;
>         unsigned int expiredRequestId_;
>         unsigned int nbuffers_;
> @@ -126,8 +129,19 @@ int FenceTest::init()
>                 return TestFail;
>         }
>  
> -       signalledRequestId_ = nbuffers_ - 2;
> -       expiredRequestId_ = nbuffers_ - 1;
> +       completedRequestId_ = 0;
> +
> +       /*
> +        * All but two requests are queued without a fence. Request
> +        * expiredRequestId_ will be queued with a fence that we won't signal
> +        * (which is then expected to expire), and request signalledRequestId_
> +        * will be queued with a fence that gets signalled. Select nbuffers_
> +        * and nbuffers_ * 2 for those two requests, to space them by a few
> +        * frames while still not requiring a long time for the test to
> +        * complete.
> +        */
> +       expiredRequestId_ = nbuffers_;
> +       signalledRequestId_ = nbuffers_ * 2;

Thank you - that's /far/ clearer as to what's going on now and why
there's a * 2.

Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>

>  
>         return TestPass;
>  }
> @@ -189,16 +203,16 @@ void FenceTest::requestRequeue(Request *request)
>         const Request::BufferMap &buffers = request->buffers();
>         const Stream *stream = buffers.begin()->first;
>         FrameBuffer *buffer = buffers.begin()->second;
> -       uint64_t cookie = request->cookie();
>  
>         request->reuse();
>  
> -       if (cookie == signalledRequestId_ && setFence_) {
> +       if (completedRequestId_ == signalledRequestId_ - nbuffers_ && setFence_) {
>                 /*
> -                * The second time this request is queued add a fence to it.
> -                *
> -                * The main loop signals it by using a timer to write to the
> -                * efd2_ file descriptor before the fence expires.
> +                * This is the request that will be used to test fence
> +                * signalling when it completes next time. Add a fence to it,
> +                * using efd2_. The main loop will signal the fence by using a
> +                * timer to write to the efd2_ file descriptor before the fence
> +                * expires.
>                  */
>                 std::unique_ptr<Fence> fence =
>                         std::make_unique<Fence>(std::move(eventFd2_));
> @@ -213,16 +227,15 @@ void FenceTest::requestRequeue(Request *request)
>  
>  void FenceTest::requestComplete(Request *request)
>  {
> -       uint64_t cookie = request->cookie();
> -       completedRequest_ = cookie;
> +       completedRequestId_++;
>  
>         /*
> -        * The last request is expected to fail as its fence has not been
> -        * signaled.
> +        * Request expiredRequestId_ is expected to fail as its fence has not
> +        * been signalled.
>          *
>          * Validate the fence status but do not re-queue it.
>          */
> -       if (cookie == expiredRequestId_) {
> +       if (completedRequestId_ == expiredRequestId_) {
>                 if (validateExpiredRequest(request) != TestPass)
>                         expectedCompletionResult_ = false;
>  
> @@ -230,7 +243,7 @@ void FenceTest::requestComplete(Request *request)
>                 return;
>         }
>  
> -       /* Validate all requests but the last. */
> +       /* Validate all other requests. */
>         if (validateRequest(request) != TestPass) {
>                 expectedCompletionResult_ = false;
>  
> @@ -271,7 +284,7 @@ int FenceTest::run()
>                 }
>  
>                 int ret;
> -               if (i == expiredRequestId_) {
> +               if (i == expiredRequestId_ - 1) {
>                         /* This request will have a fence, and it will expire. */
>                         std::unique_ptr<Fence> fence =
>                                 std::make_unique<Fence>(std::move(eventFd_));
> @@ -318,11 +331,13 @@ int FenceTest::run()
>         Timer timer;
>         timer.start(1000ms);
>         while (timer.isRunning() && expectedCompletionResult_) {
> -               if (completedRequest_ == signalledRequestId_ && setFence_)
> +               if (completedRequestId_ == signalledRequestId_ - 1 && setFence_)
>                         /*
> -                        * signalledRequestId_ has just completed and it has
> -                        * been re-queued with a fence. Start the timer to
> -                        * signal the fence in 10 msec.
> +                        * The request just before signalledRequestId_ has just
> +                        * completed. Request signalledRequestId_ has been
> +                        * queued with a fence, and libcamera is likely already
> +                        * waiting on the fence, or will soon. Start the timer
> +                        * to signal the fence in 10 msec.
>                          */
>                         fenceTimer.start(10ms);
>  
> -- 
> Regards,
> 
> Laurent Pinchart
>