[PATCH v1 3/6] test: fence: Fix race condition

[Kieran Bingham]

Quoting Laurent Pinchart (2024-04-25 00:42:21)
> 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, is zero-based and wraps around at

Do you mean "which is zero-based"

> 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>
> ---
>  test/fence.cpp | 29 ++++++++++++++---------------
>  1 file changed, 14 insertions(+), 15 deletions(-)
> 
> diff --git a/test/fence.cpp b/test/fence.cpp
> index e6f79d2faa21..7949bfbb176b 100644
> --- a/test/fence.cpp
> +++ b/test/fence.cpp
> @@ -57,8 +57,7 @@ private:
>         bool expectedCompletionResult_ = true;
>         bool setFence_ = true;
>  
> -       unsigned int completedRequest_;
> -
> +       unsigned int completedRequestId_;
>         unsigned int signalledRequestId_;
>         unsigned int expiredRequestId_;
>         unsigned int nbuffers_;
> @@ -126,8 +125,9 @@ int FenceTest::init()
>                 return TestFail;
>         }
>  
> -       signalledRequestId_ = nbuffers_ - 2;
> -       expiredRequestId_ = nbuffers_ - 1;
> +       completedRequestId_ = 0;
> +       expiredRequestId_ = nbuffers_;
> +       signalledRequestId_ = nbuffers_ * 2;

I don't (yet) understand hy this is nbuffers_ * 2 ... while before it
was nbuffers_ - 2 ... that stands out a lot ... but maybe reading
further down will explain.

Maybe that makes it worthy of a comment above to say why those values /
id targets are chosen.

I've read down, and the * 2 isn't clearer. Perhaps it's just to make
sure that enough requests are queued?

>  
>         return TestPass;
>  }
> @@ -189,16 +189,16 @@ 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_));
> @@ -213,8 +213,7 @@ 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
> @@ -222,7 +221,7 @@ void FenceTest::requestComplete(Request *request)
>          *
>          * Validate the fence status but do not re-queue it.
>          */
> -       if (cookie == expiredRequestId_) {
> +       if (completedRequestId_ == expiredRequestId_) {
>                 if (validateExpiredRequest(request) != TestPass)
>                         expectedCompletionResult_ = false;
>  
> @@ -271,7 +270,7 @@ 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_));
> @@ -318,7 +317,7 @@ 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
> -- 
> Regards,
> 
> Laurent Pinchart
>