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

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Message-ID: <20140731164918.GC7842@localhost.localdomain>
Date:	Thu, 31 Jul 2014 18:49:20 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Oleg Nesterov <oleg@...hat.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 Thu, Jul 31, 2014 at 06:42:46PM +0200, Oleg Nesterov wrote:
> 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.

That's a point too! You guys both convinced me! ;-)

> 
> > > 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.

No because all running tasks have this flag set when context tracking is
enabled. And context tracking can't be disabled on runtime.
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