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Date: Sun, 15 Mar 2009 01:02:39 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Ingo Molnar <mingo@elte.hu>
Cc: Frederic Weisbecker <fweisbec@gmail.com>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Steven Rostedt <rostedt@goodmis.org>, tglx@linutronix.de,
       Jason Baron <jbaron@redhat.com>, "Frank Ch. Eigler" <fche@redhat.com>,
       Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
       KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
       Lai Jiangshan <laijs@cn.fujitsu.com>,
       Jiaying Zhang <jiayingz@google.com>, Michael Rubin <mrubin@google.com>,
       Martin Bligh <mbligh@google.com>, Michael Davidson <md@google.com>
Subject: Re: [PATCH 1/2 v2] tracing/syscalls: core infrastructure for
 syscalls tracing
Message-Id: <20090315010239.d4a8b3ac.akpm@linux-foundation.org>
In-Reply-To: <20090315045904.GA20949@elte.hu>
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On Sun, 15 Mar 2009 05:59:04 +0100 Ingo Molnar <mingo@elte.hu> wrote:

> 
> * Andrew Morton <akpm@linux-foundation.org> wrote:
> 
> > On Fri, 13 Mar 2009 15:42:11 +0100 Frederic Weisbecker <fweisbec@gmail.com> wrote:
> > 
> > > +void start_ftrace_syscalls(void)
> > > +{
> > > +	unsigned long flags;
> > > +	struct task_struct *g, *t;
> > > +
> > > +	if (atomic_inc_return(&refcount) != 1)
> > > +		goto out;
> > > +
> > > +	arch_init_ftrace_syscalls();
> > > +	read_lock_irqsave(&tasklist_lock, flags);
> > > +
> > > +	do_each_thread(g, t) {
> > > +		set_tsk_thread_flag(t, TIF_SYSCALL_FTRACE);
> > > +	} while_each_thread(g, t);
> > > +
> > > +	read_unlock_irqrestore(&tasklist_lock, flags);
> > > +out:
> > > +	atomic_dec(&refcount);
> > > +}
> > > +
> > > +void stop_ftrace_syscalls(void)
> > > +{
> > > +	unsigned long flags;
> > > +	struct task_struct *g, *t;
> > > +
> > > +	if (atomic_dec_return(&refcount))
> > > +		goto out;
> > > +
> > > +	read_lock_irqsave(&tasklist_lock, flags);
> > > +
> > > +	do_each_thread(g, t) {
> > > +		clear_tsk_thread_flag(t, TIF_SYSCALL_FTRACE);
> > > +	} while_each_thread(g, t);
> > > +
> > > +	read_unlock_irqrestore(&tasklist_lock, flags);
> > > +out:
> > > +	atomic_inc(&refcount);
> > > +}
> > 
> > What is this `refcount' thing trying to do?  afacit it does 
> > not prevent the two loops from running concurrently and making 
> > a mess.
> > 
> > If it _is_ trying to prevent that from happening, then why not 
> > use plain old mutex_lock()?
> 
> yeah - already commented about that to Frederic over IRC. A 
> plain flag, checked inside the tasklist lock is more than 
> enough.
> 

That would require write_lock().