Re: [PATCH hwc v2 02/18] drm_hwcomposer: vsyncworker: Fix deadlock on exit path

[Robert Foss <robert.foss@collabora.com>]

Hey Alexandru,

Feel free my SoB on this patch:
Signed-off-by: Robert Foss <robert.foss@collabora.com>


Rob.


On 04/11/2018 05:22 PM, Alexandru Gheorghe wrote:
> vsyncworker::Routine assumes that when -EINTR is returned by
> WaitForSignalOrExitLocked the lock as been released, which is not
> true, so it hangs if a vsyncworker is never enabled and Exit is
> called.
> 
> There are two code paths in WaitForSignalOrExitLocked that return
> -EINTR, one releases the lock the other doesn't.
> Looking at the clients of WaitForSignalOrExitLocked all assume the lock
> is still held, except vsyncworker::Routine.
> So, the proper fix needs two changes:
> - Make WaitForSignalOrExitLocked consistent and always hold the lock
>    when exiting.
> - Release lock in vsynworker::Routine on all code paths.
> 
> Signed-off-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
> ---
>   vsyncworker.cpp | 1 +
>   worker.cpp      | 6 +++---
>   2 files changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/vsyncworker.cpp b/vsyncworker.cpp
> index 3bfe4be..7c0c741 100644
> --- a/vsyncworker.cpp
> +++ b/vsyncworker.cpp
> @@ -120,6 +120,7 @@ void VSyncWorker::Routine() {
>     if (!enabled_) {
>       ret = WaitForSignalOrExitLocked();
>       if (ret == -EINTR) {
> +      Unlock();
>         return;
>       }
>     }
> diff --git a/worker.cpp b/worker.cpp
> index da6c580..5b351e0 100644
> --- a/worker.cpp
> +++ b/worker.cpp
> @@ -66,13 +66,13 @@ int Worker::WaitForSignalOrExitLocked(int64_t max_nanoseconds) {
>       ret = -ETIMEDOUT;
>     }
>   
> +  // release leaves mutex locked when going out of scope
> +  lk.release();
> +
>     // exit takes precedence on timeout
>     if (should_exit())
>       ret = -EINTR;
>   
> -  // release leaves mutex locked when going out of scope
> -  lk.release();
> -
>     return ret;
>   }
>   
> 
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