Re: [dpdk-dev] [PATCH] net/tap: avoid using SIGIO

[Stephen Hemminger <stephen@networkplumber.org>]

On Mon, 27 Jul 2020 15:19:32 +0200
Morten Brørup <mb@smartsharesystems.com> wrote:

> > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Stephen Hemminger
> > Sent: Wednesday, July 15, 2020 1:58 AM
> > 
> > SIGIO maybe used by application, instead choose another rt-signal.
> > Linux allows any signal to be used for signal based IO.
> > Search for an unused signal in the available rt-signal range.  
> 
> Just an observation. Feel free to ignore at your convenience:
> 
> The problem is the same as for SIGIO if the application sets up its own signal handler after this, and uses some hardcoded rt-signal that happens to be the one found to be free.
> 
> Unless the application doesn't use a hardcoded rt-signal, but also searches for an unused one.
> 
> So perhaps the "search for unused rt-signal" should be exposed as a generic support function for the application (and this driver) to use.

There is no safe way to use a signal deep inside DPDK in a driver.

This is not the kind of thing that should be exposed to the application.

The algorithm for finding an RT signal conforms to the recommended policy on the signal(7)
manual page.

       programs should never refer to real-time signals using hard-
       coded numbers, but instead should always refer to real-time signals
       using the notation SIGRTMIN+n, and include suitable (run-time) checks
       that SIGRTMIN+n does not exceed SIGRTMAX.

The application should be following the proscribed policy on the man page.
If it doesn't it is broken.