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

[Andrew Morton <akpm@linux-foundation.org>]

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()?

If the code is or some reason correct then it certainly is not sufficiently
obvious to be let uncommented.