Fix: additional compiler barriers for procname context

[lttng-ust.git] / liblttng-ust / lttng-context-procname.c

/*
 * lttng-context-procname.c
 *
 * LTTng UST procname context.
 *
 * Copyright (C) 2009-2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; only
 * version 2.1 of the License.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */

#define _LGPL_SOURCE
#include <lttng/ust-events.h>
#include <lttng/ust-tracer.h>
#include <lttng/ringbuffer-config.h>
#include <urcu/tls-compat.h>
#include <assert.h>
#include "compat.h"

/* Maximum number of nesting levels for the procname cache. */
#define PROCNAME_NESTING_MAX	2

/*
 * We cache the result to ensure we don't trigger a system call for
 * each event.
 * Upon exec, procname changes, but exec takes care of throwing away
 * this cached version.
 * The procname can also change by calling prctl(). The procname should
 * be set for a thread before the first event is logged within this
 * thread.
 */
typedef char procname_array[PROCNAME_NESTING_MAX][17];

static DEFINE_URCU_TLS(procname_array, cached_procname);

static DEFINE_URCU_TLS(int, procname_nesting);

static inline
char *wrapper_getprocname(void)
{
	int nesting = CMM_LOAD_SHARED(URCU_TLS(procname_nesting));

	if (caa_unlikely(nesting >= PROCNAME_NESTING_MAX))
		return "<unknown>";
	if (caa_unlikely(!URCU_TLS(cached_procname)[nesting][0])) {
		CMM_STORE_SHARED(URCU_TLS(procname_nesting), nesting + 1);
		/* Increment nesting before updating cache. */
		cmm_barrier();
		lttng_ust_getprocname(URCU_TLS(cached_procname)[nesting]);
		URCU_TLS(cached_procname)[nesting][LTTNG_UST_PROCNAME_LEN - 1] = '\0';
		/* Decrement nesting after updating cache. */
		cmm_barrier();
		CMM_STORE_SHARED(URCU_TLS(procname_nesting), nesting);
	}
	return URCU_TLS(cached_procname)[nesting];
}

/* Reset should not be called from a signal handler. */
void lttng_context_procname_reset(void)
{
	CMM_STORE_SHARED(URCU_TLS(cached_procname)[1][0], '\0');
	CMM_STORE_SHARED(URCU_TLS(procname_nesting), 1);
	CMM_STORE_SHARED(URCU_TLS(cached_procname)[0][0], '\0');
	CMM_STORE_SHARED(URCU_TLS(procname_nesting), 0);
}

static
size_t procname_get_size(struct lttng_ctx_field *field, size_t offset)
{
	size_t size = 0;

	size += LTTNG_UST_PROCNAME_LEN;
	return size;
}

static
void procname_record(struct lttng_ctx_field *field,
		 struct lttng_ust_lib_ring_buffer_ctx *ctx,
		 struct lttng_channel *chan)
{
	char *procname;

	procname = wrapper_getprocname();
	chan->ops->event_write(ctx, procname, LTTNG_UST_PROCNAME_LEN);
}

static
void procname_get_value(struct lttng_ctx_field *field,
		struct lttng_ctx_value *value)
{
	char *procname;

	procname = wrapper_getprocname();
	value->u.str = procname;
}

int lttng_add_procname_to_ctx(struct lttng_ctx **ctx)
{
	struct lttng_ctx_field *field;

	field = lttng_append_context(ctx);
	if (!field)
		return -ENOMEM;
	if (lttng_find_context(*ctx, "procname")) {
		lttng_remove_context_field(ctx, field);
		return -EEXIST;
	}
	field->event_field.name = "procname";
	field->event_field.type.atype = atype_array;
	field->event_field.type.u.array.elem_type.atype = atype_integer;
	field->event_field.type.u.array.elem_type.u.basic.integer.size = sizeof(char) * CHAR_BIT;
	field->event_field.type.u.array.elem_type.u.basic.integer.alignment = lttng_alignof(char) * CHAR_BIT;
	field->event_field.type.u.array.elem_type.u.basic.integer.signedness = lttng_is_signed_type(char);
	field->event_field.type.u.array.elem_type.u.basic.integer.reverse_byte_order = 0;
	field->event_field.type.u.array.elem_type.u.basic.integer.base = 10;
	field->event_field.type.u.array.elem_type.u.basic.integer.encoding = lttng_encode_UTF8;
	field->event_field.type.u.array.length = LTTNG_UST_PROCNAME_LEN;
	field->get_size = procname_get_size;
	field->record = procname_record;
	field->get_value = procname_get_value;
	lttng_context_update(*ctx);
	return 0;
}

/*
 * Force a read (imply TLS fixup for dlopen) of TLS variables.
 */
void lttng_fixup_procname_tls(void)
{
	asm volatile ("" : : "m" (URCU_TLS(cached_procname)[0]));
}