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SubjectRe: [patch 02/17] Kernel Tracepoints

[Added Paul McKenney to CC]

On Tue, 15 Jul 2008, Mathieu Desnoyers wrote:
> +++ linux-2.6-lttng/include/linux/tracepoint.h 2008-07-15 17:35:19.000000000 -0400
> @@ -0,0 +1,127 @@
> +#ifndef _LINUX_TRACEPOINT_H
> +#define _LINUX_TRACEPOINT_H
> +
> +/*
> + * Kernel Tracepoint API.
> + *
> + * See Documentation/tracepoint.txt.
> + *
> + * (C) Copyright 2008 Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
> + *
> + * Heavily inspired from the Linux Kernel Markers.
> + *
> + * This file is released under the GPLv2.
> + * See the file COPYING for more details.
> + */
> +
> +#include <linux/types.h>
> +#include <linux/rcupdate.h>
> +
> +struct module;
> +struct tracepoint;
> +
> +struct tracepoint {
> + const char *name; /* Tracepoint name */
> + int state; /* State. */
> + void **funcs;
> +} __attribute__((aligned(8)));
> +
> +
> +#define TPPROTO(args...) args
> +#define TPARGS(args...) args
> +
> +#ifdef CONFIG_TRACEPOINTS
> +
> +/*
> + * it_func[0] is never NULL because there is at least one element in the array
> + * when the array itself is non NULL.
> + */
> +#define __DO_TRACE(tp, proto, args) \
> + do { \
> + void **it_func; \
> + \
> + rcu_read_lock_sched(); \
> + it_func = rcu_dereference((tp)->funcs); \
> + if (it_func) { \
> + do { \
> + ((void(*)(proto))(*it_func))(args); \
> + } while (*(++it_func)); \

OK, I still don't understand the concept of the rcu_dereference, but why
is it needed for the first assignment of it_func but not the ++? Is it
only needed with the (tp)->funcs?

-- Steve


> + } \
> + rcu_read_unlock_sched(); \
> + } while (0)



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