linux-kernel - Re: [PATCH RESEND 2/2] tracing: syscall_regfunc() should not skip kernel threads

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Date:	Thu, 10 Apr 2014 11:08:48 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Hendrik Brueckner <brueckner@...ux.vnet.ibm.com>
Subject: Re: [PATCH RESEND 2/2] tracing: syscall_regfunc() should not skip
 kernel threads

On Thu, 10 Apr 2014 16:46:55 +0200
Oleg Nesterov <oleg@...hat.com> wrote:


> > I mean, the tracepoint is
> > activated usually by humans, and if they enabled it just as a usermode
> > helper is activated, and those are really fast to run, do we even care
> > if it is missed?
> 
> A user space task spawned by call_usermodehelper() can do everything, it
> can run forever.

Sounds nasty ;-)

> 
> > Now, if tracing is on and we need to set the flag, that should take the
> > task list lock to make sure that we don't miss clearing it. Missing the
> > set isn't a big deal, but missing the clearing of the flag is.
> >
> > void tracepoint_check_syscalls(void)
> > {
> > 	if (!sys_tracepoint_refcount)
> > 		return;
> >
> > 	read_lock(&tasklist_lock);
> > 	/* Make sure it wasn't cleared since taking the lock */
> > 	if (sys_tracepoint_refcount)
> > 		set_tsk_thread_flag(current, TIF_SYSCALL_TRACEPOINT);
> > 	read_unlock(&tasklist_lock);
> > }
> 
> And how this can help to avoid the race? We need write_lock_irq().

But you chopped off the last part. Where I replaced tasklist_lock with
a tracepoint specific lock that would synchronize
sys_tracepoint_refcount with the setting of the flags.


> 
> Perhaps I missed something... and I simply do not understand why do you
> want to do this.

Because I'm being an ass ;-)

The real reason I'm doing this debate is more to find out exactly what
the problems are. A learning exercise if you will. I just don't want to
add a regression, as Hendrik (which I just Cc'd) added the commit for a
reason. Perhaps you are correct that we should just go back to the way
things were.

Hendrik, we are debating about removing
cc3b13c11c567c69a6356be98d0c03ff11541d5c as it stops
call_usermodehelper tasks from tracing their syscalls.

If Hendrik has no problems with this, neither do I.

-- Steve
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