linux-kernel - Re: [PATCH v4 2/5] x86,entry: Only call user_exit if TIF

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Date:	Thu, 31 Jul 2014 18:42:46 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Kees Cook <keescook@...omium.org>,
	Will Drewry <wad@...omium.org>, X86 ML <x86@...nel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Linux MIPS Mailing List <linux-mips@...ux-mips.org>,
	linux-arch <linux-arch@...r.kernel.org>,
	LSM List <linux-security-module@...r.kernel.org>,
	Alexei Starovoitov <ast@...mgrid.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v4 2/5] x86,entry: Only call user_exit if TIF_NOHZ

On 07/31, Frederic Weisbecker wrote:
>
> On Wed, Jul 30, 2014 at 10:23:34AM -0700, Andy Lutomirski wrote:
> >
> > At the end of the day, the syscall slowpath code calls a bunch of
> > functions depending on what TIF_XYZ flags are set.  As long as it's
> > structured like "if (TIF_A) do_a(); if (TIF_B) do_b();" or something
> > like that, it's comprehensible.  But once random functions with no
> > explicit flag checks or comments start showing up, it gets confusing.
>
> Yeah that's a point. I don't mind much the TIF_NOHZ test if you like.

And in my opinion

	if (work & TIF_XYZ)
		user_exit();

looks even more confusing. Because, once again, TIF_XYZ is not the
reason to call user_exit().

Not to mention this adds a minor performance penalty.

> > If it's indeed all-or-nothing, I could remove the check and add a
> > comment.  But please keep in mind that, currently, the slow path is
> > *slow*, and my patches only improve the entry case.  So enabling
> > context tracking on every task will hurt.
>
> That's what we do anyway. I haven't found a safe way to enabled context tracking
> without tracking all CPUs.

And if we change this, then the code above becomes racy. The state of
TIF_XYZ can be changed right after the check. OK, it is racy anyway ;)
but still this adds more confusion.

I feel that TIF_XYZ must die. But yes, yes, I know that it is very simple
to say this. And no, so far I do not know how we can improve this all.


But again, again, I won't insist. Just another "can't resist" email,
please ignore.

Oleg.

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