Hi,
On Mon, 19 Apr 2021 at 11:48, Marek Olšák maraeo@gmail.com wrote:
Deadlock mitigation to recover from segfaults:
- The kernel knows which process is obliged to signal which fence. This
information is part of the Present request and supplied by userspace.
- If the producer crashes, the kernel signals the submit fence, so that
the consumer can make forward progress.
- If the consumer crashes, the kernel signals the return fence, so that
the producer can reclaim the buffer.
- A GPU hang signals all fences. Other deadlocks will be handled like GPU
hangs.
Another thought: with completely arbitrary userspace fencing, none of this is helpful either. If the compositor can't guarantee that a hostile client has submitted a fence which will never be signaled, then it won't be waiting on it, so it already needs infrastructure to handle something like this. That already handles the crashed-client case, because if the client crashes, then its connection will be dropped, which will trigger the compositor to destroy all its resources anyway, including any pending waits.
GPU hangs also look pretty similar; it's an infinite wait, until the client resubmits a new buffer which would replace (& discard) the old.
So signal-fence-on-process-exit isn't helpful and doesn't provide any extra reliability; it in fact probably just complicates things.
Cheers, Daniel