Re: [Qemu-devel] patch to swap SIGRTMIN + 1 and SIGRTMAX - 1

From: Laurent Vivier <laurent@vivier.eu>
To: Josh Kunz <jkz@google.com>,
	Aleksandar Markovic <aleksandar.markovic@rt-rk.com>,
	milos.stojanovic@rt-rk.com
Cc: Peter Maydell <peter.maydell@linaro.org>,
	qemu-trivial@nongnu.org, riku.voipio@iki.fi,
	qemu-devel@nongnu.org, marlies.ruck@gmail.com,
	Aleksandar Markovic <amarkovic@wavecomp.com>,
	Shu-Chun Weng <scw@google.com>
Subject: Re: [Qemu-devel] patch to swap SIGRTMIN + 1 and SIGRTMAX - 1
Date: Wed, 28 Aug 2019 10:51:52 +0200	[thread overview]
Message-ID: <6542ac57-6b06-1337-825b-dd1187accd15@vivier.eu> (raw)
In-Reply-To: <CADgy-2va2xVmO_a1gDwg+zkpNcLJTW5D1j=2kk1TnRMxyPaLMg@mail.gmail.com>

Le 26/08/2019 à 23:10, Josh Kunz a écrit :
> On Wed, Aug 21, 2019 at 2:28 AM Laurent Vivier <laurent@vivier.eu
> <mailto:laurent@vivier.eu>> wrote:
> 
>     Le 19/08/2019 à 23:46, Josh Kunz via Qemu-devel a écrit :
>     > Hi all,
>     >
>     > I have also experienced issues with SIGRTMIN + 1, and am interested in
>     > moving this patch forwards. Anything I can do here to help? Would the
>     > maintainers prefer myself or Marli re-submit the patch?
>     >
>     > The Go issue here seems particularly sticky. Even if we update the Go
>     > runtime, users may try and run older binaries built with older
>     versions of
>     > Go for quite some time (months? years?). Would it be better to
>     hide this
>     > behind some kind of build-time flag
>     (`--enable-sigrtmin-plus-one-proxy` or
>     > something), so that some users can opt-in, but older binaries
>     still work as
>     > expected?
>     >
>     > Also, here is a link to the original thread this message is in
>     reply to
>     > in-case my mail-client doesn't set up the reply properly:
>     > https://lists.nongnu.org/archive/html/qemu-devel/2019-07/msg01303.html
> 
>     The problem here is we break something to fix something else.
> 
>     I'm wondering if the series from Aleksandar Markovic, "linux-user:
>     Support signal passing for targets having more signals than host" [1]
>     can fix the problem in a better way?
> 
> 
> That patch[1] (which I'll refer to as the MUX patch to avoid confusion)
> does not directly fix the issue addressed by this patch (re-wiring
> SIGRTMIN+1), but since it basically implements generic signal
> multiplexing, it could be re-worked to address this case as well. The
> way it handles `si_code` spooks me a little bit. It could easily be
> broken by a kernel version change, and such a breakage could be hard to
> detect or lead to surprising results. Other than that, it looks like a
> reasonable implementation.
> 
> That said, overall, fixing the SIGRTMIN+1 issue using a more-generic
> signal-multiplexing mechanism doesn't seem *that* much better to me. It
> adds a lot of complexity, and only saves a single signal (assuming glibc
> doesn't add more reserved signals). The "big win" is additional
> emulation features, like those introduced in MUX patch (being able to
> utilize signals outside of the host range). If having those features in
> QEMU warrants the additional complexity, then re-working this patch
> on-top of that infrastructure seems like a good idea.
> 
> If the maintainers want to go down that route, then I would be happy to
> re-work this patch utilizing the infrastructure from the MUX patch.
> Unfortunately it will require non-trivial changes, so it may be best to
> wait until that patch is merged. I could also provide a patch "on top
> of" the MUX patch, if that's desired/more convenient.
> 
> Just one last note, if you do decide to merge the MUX patch, then it
> would be best to use SIGRTMAX (instead of SIGRTMAX-1) as the
> multiplexing signal if possible, to avoid breaking go binaries.
> 

Personally, I prefer a solution that breaks nothing.

Aleksandar, Milos,

do you have an updated version of you series "Support signal passing for
targets having more signals than host"?

Thanks,
Laurent