linux-kernel - Re: syscall_regfunc() && TIF_SYSCALL

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Date:	Sat, 31 Mar 2012 22:45:01 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Ingo Molnar <mingo@...hat.com>, Jason Baron <jbaron@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: syscall_regfunc() && TIF_SYSCALL_TRACEPOINT

On 03/30, Steven Rostedt wrote:
>
> On Fri, 2012-03-30 at 22:15 +0200, Oleg Nesterov wrote:
>
> > But I don't really understand why do you think that "clear" is more
> > important.
>
> They are both important. But as I tend to consider performance when
> tracing is off as critical, I'm more concerned about that. But both must
> be fixed, because not reporting traces can confuse a developer.

Ah, got it, thanks.

I was going to send the simple patch we discussed, but suddenly I
realized that I have another question.

Why do we want to filter out the kernel threads in syscall_regfunc?

>From cc3b13c1 "tracing: Don't trace kernel thread syscalls"

	 then it has no effect to trace the kernel thread calls
	 to syscalls in that path.
	 Setting the TIF_SYSCALL_TRACEPOINT flag is then useless for these.

OK, but then it doesn't hurt? Or is there another reason why
TIF_SYSCALL_TRACEPOINT is not desirable on kthread?

The problem is ____call_usermodehelper() which execs the user-space
task. This clears PF_KTHREAD (sets ->mm), but obviously if
sys_tracepoint_refcount != 0 this is too late.

So what do you think we should do,

	- keep this check

	- remove it

	- remove it in a separate patch

	- add the "sync with sys_tracepoint_refcount" hook
	  before kernel_execve()

?

Oleg.

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