Re: [PATCH 1/2 v2] tracing/syscalls: core infrastructure for syscalls tracing

[Frederic Weisbecker <fweisbec@gmail.com>]

On Sun, Mar 15, 2009 at 01:02:39AM -0700, Andrew Morton wrote:
> 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().


Hm, I made a mistake in this code.
What I wanted to do is to allow several tracing callsites to ask
for syscalls tracing and to only disable it when the last user releases
it (and avoid the iterate the tasklist one more time while the flag
is already set).

So I shouldn't increment it back when there is already one user,
I should only exit without doing anything.

But I still need a lock to prevent from concurrent clear/set flag
on the tasks when the last user is disabling syscall tracing and a new one
requests it.

I need a mutex here. I'm preparing it.

Thanks.