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

Andrew Morton
2009-03-15T08:15:48Z

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().