On Sat, Nov 28, 2020 at 7:07 PM <rao.shoaib@xxxxxxxxxx> wrote: > > From: Rao Shoaib <rao.shoaib@xxxxxxxxxx> > > We have a use case where thousands of processes connect locally to a > database and issue queries that are serviced by a pool of threads. > Communication is done over AF_UNIX sockets. Currently, there is no way > for the submitter to signal the servicing thread about an urgent > condition such as abandoning the query. Sure there is. You could close() the socket. You could send() a message saying that your query wants to be canceled. You could use a second socket for this if it's somehow inconvenient to do it with a single socket. > This patch addresses that > requirement by adding support for MSG_OOB flag for AF_UNIX sockets. > On receipt of such a flag, the kernel sends a SIGURG to the peer. SIGURG is a horrible interface. It doesn't scale -- signals are slow, and they give no indication of *which* socket has urgent data. Aside from this, the "urgent" interface itself is nuts -- even the TCP RFCs and implementations seem unable to agree on exactly how to tell *which* data is urgent. At least epoll might be a scalable way to tell which socket has urgent data, but that's not what your patch description seems to be talking about. Oh yeah, SIGURG simply doesn't work across privilege boundaries. I'm also a bit nervous that there might be some program out there that expects SIGIO but not SIGURG on unix sockets, and you're breaking them. --Andy
