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

Ingo Molnar
2009-03-15T05:00:10Z

* 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.

Ingo