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

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Date:	Wed, 30 Jul 2014 18:43:46 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
	Will Drewry <wad@...omium.org>, x86@...nel.org,
	linux-arm-kernel@...ts.infradead.org, linux-mips@...ux-mips.org,
	linux-arch@...r.kernel.org, linux-security-module@...r.kernel.org,
	Alexei Starovoitov <ast@...mgrid.com>, hpa@...or.com
Subject: Re: [PATCH v4 2/5] x86,entry: Only call user_exit if TIF_NOHZ

On Tue, Jul 29, 2014 at 09:32:32PM +0200, Oleg Nesterov wrote:
> On 07/28, Andy Lutomirski wrote:
> >
> > @@ -1449,7 +1449,12 @@ long syscall_trace_enter(struct pt_regs *regs)
> >  {
> >  	long ret = 0;
> >  
> > -	user_exit();
> > +	/*
> > +	 * If TIF_NOHZ is set, we are required to call user_exit() before
> > +	 * doing anything that could touch RCU.
> > +	 */
> > +	if (test_thread_flag(TIF_NOHZ))
> > +		user_exit();
> 
> Personally I still think this change just adds more confusion, but I leave
> this to you and Frederic.
> 
> It is not that "If TIF_NOHZ is set, we are required to call user_exit()", we
> need to call user_exit() just because we enter the kernel. TIF_NOHZ is just
> the implementation detail which triggers this slow path.
> 
> At least it should be correct, unless I am confused even more than I think.

Agreed, Perhaps the confusion is on the syscall_trace_enter() name which suggests
this is only about tracing? syscall_slowpath_enter() could be an alternative.
But that's still tracing in a general sense so...
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