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

[Sean Paul <seanpaul@chromium.org>]

On Wed, Apr 11, 2018 at 04:22:13PM +0100, 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();
> -

I'm not sure why this chunk makes a difference? If the above was
"return -EINTR;" it would, but it's just assigning ret.

Sean

>    return ret;
>  }
>  
> -- 
> 2.7.4
> 

-- 
Sean Paul, Software Engineer, Google / Chromium OS
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