musl - Re: Re: [RFC PATCH] x86/vdso/32: Add AT

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Date: Sat, 12 Mar 2016 18:08:56 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Rich Felker <dalias@...c.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	"musl@...ts.openwall.com" <musl@...ts.openwall.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Szabolcs Nagy <nsz@...t70.net>
Subject: Re: Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation
 helpers


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* Szabolcs Nagy <nsz@...t70.net> wrote:

> >   4. A calls cancellation point and syscall correctly executes
> >   5. Once A enables cancellation again, the cancellation propagates.
> > 
> > So I still see no problem.
> 
> i think the sticky signal design would work, but more
> complex than what we have and adds some atomic rmw ops
> into common code paths and not backward compatible.

Agreed about complexity, but note that the RMW ops shouldn't really be expensive 
here, as this should be a well-cached flag. Especially compared to:

> not using vsyscalls for cancellation-points sounds easier.

... FYI not using vsyscalls has _far_ higher cost than using well-cached RMW ops.

So ... what do you think about Linus's SA_SYNCHRONOUS approach? I think it can be 
made to work without much fuss.

There will still be different code paths on old and new kernels, but that's 
unavoidable.

Thanks,

	Ingo

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