Date: Thu, 31 Jul 2014 18:54:23 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Andy Lutomirski <luto@amacapital.net>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Kees Cook <keescook@chromium.org>, Will Drewry <wad@chromium.org>,
        X86 ML <x86@kernel.org>,
        "linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
        Linux MIPS Mailing List <linux-mips@linux-mips.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        LSM List <linux-security-module@vger.kernel.org>,
        Alexei Starovoitov <ast@plumgrid.com>,
        "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH v4 2/5] x86,entry: Only call user_exit if TIF_NOHZ
Message-ID: <20140731165423.GA16800@redhat.com>
References: <cover.1406604806.git.luto@amacapital.net> <7123b2489cc5d1d5abb7bcf1364ca729cab3e6ca.1406604806.git.luto@amacapital.net> <20140729193232.GA8153@redhat.com> <20140730164344.GA27954@localhost.localdomain> <CALCETrUVaz3JFiNbyU=r3M-E9muHa1ffn7RX+_-4V_0U-hVaPw@mail.gmail.com> <20140731151630.GA7842@localhost.localdomain> <20140731164246.GA15974@redhat.com> <20140731164918.GC7842@localhost.localdomain>
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On 07/31, Frederic Weisbecker wrote:
>
> 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! ;-)

Very nice, now I know that you can agree with 2 opposite opinions at
the same time ;)

> > > > 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.

Yes, yes, please note that I said "if we change this".

Oleg.

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