liblttng-ust/lttng-context-vtid.c

   1 /*
   2  * lttng-context-vtid.c
   3  *
   4  * LTTng UST vtid context.
   5  *
   6  * Copyright (C) 2009-2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   7  *
   8  * This library is free software; you can redistribute it and/or
   9  * modify it under the terms of the GNU Lesser General Public
  10  * License as published by the Free Software Foundation; only
  11  * version 2.1 of the License.
  12  *
  13  * This library is distributed in the hope that it will be useful,
  14  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  15  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  16  * Lesser General Public License for more details.
  17  *
  18  * You should have received a copy of the GNU Lesser General Public
  19  * License along with this library; if not, write to the Free Software
  20  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  21  */
  22
  23 #define _LGPL_SOURCE
  24 #include <stddef.h>
  25 #include <sys/types.h>
  26 #include <unistd.h>
  27 #include <lttng/ust-events.h>
  28 #include <lttng/ust-tracer.h>
  29 #include <lttng/ringbuffer-config.h>
  30 #include <lttng/ust-tid.h>
  31 #include <urcu/tls-compat.h>
  32 #include "lttng-tracer-core.h"
  33
  34 /*
  35  * We cache the result to ensure we don't trigger a system call for
  36  * each event.
  37  */
  38 static DEFINE_URCU_TLS(pid_t, cached_vtid);
  39
  40 /*
  41  * Upon fork or clone, the TID assigned to our thread is not the same as
  42  * we kept in cache. Luckily, we are the only thread surviving in the
  43  * child process, so we can simply clear our cached version.
  44  */
  45 void lttng_context_vtid_reset(void)
  46 {
  47         CMM_STORE_SHARED(URCU_TLS(cached_vtid), 0);
  48 }
  49
  50 static
  51 size_t vtid_get_size(struct lttng_ctx_field *field, size_t offset)
  52 {
  53         size_t size = 0;
  54
  55         size += lib_ring_buffer_align(offset, lttng_alignof(pid_t));
  56         size += sizeof(pid_t);
  57         return size;
  58 }
  59
  60 static inline
  61 pid_t wrapper_getvtid(void)
  62 {
  63         pid_t vtid;
  64
  65         vtid = CMM_LOAD_SHARED(URCU_TLS(cached_vtid));
  66         if (caa_unlikely(!vtid)) {
  67                 vtid = lttng_gettid();
  68                 CMM_STORE_SHARED(URCU_TLS(cached_vtid), vtid);
  69         }
  70         return vtid;
  71 }
  72
  73 static
  74 void vtid_record(struct lttng_ctx_field *field,
  75                  struct lttng_ust_lib_ring_buffer_ctx *ctx,
  76                  struct lttng_channel *chan)
  77 {
  78         pid_t vtid = wrapper_getvtid();
  79
  80         lib_ring_buffer_align_ctx(ctx, lttng_alignof(vtid));
  81         chan->ops->event_write(ctx, &vtid, sizeof(vtid));
  82 }
  83
  84 static
  85 void vtid_get_value(struct lttng_ctx_field *field,
  86                 struct lttng_ctx_value *value)
  87 {
  88         value->u.s64 = wrapper_getvtid();
  89 }
  90
  91 int lttng_add_vtid_to_ctx(struct lttng_ctx **ctx)
  92 {
  93         struct lttng_ctx_field *field;
  94
  95         field = lttng_append_context(ctx);
  96         if (!field)
  97                 return -ENOMEM;
  98         if (lttng_find_context(*ctx, "vtid")) {
  99                 lttng_remove_context_field(ctx, field);
 100                 return -EEXIST;
 101         }
 102         field->event_field.name = "vtid";
 103         field->event_field.type.atype = atype_integer;
 104         field->event_field.type.u.integer.size = sizeof(pid_t) * CHAR_BIT;
 105         field->event_field.type.u.integer.alignment = lttng_alignof(pid_t) * CHAR_BIT;
 106         field->event_field.type.u.integer.signedness = lttng_is_signed_type(pid_t);
 107         field->event_field.type.u.integer.reverse_byte_order = 0;
 108         field->event_field.type.u.integer.base = 10;
 109         field->event_field.type.u.integer.encoding = lttng_encode_none;
 110         field->get_size = vtid_get_size;
 111         field->record = vtid_record;
 112         field->get_value = vtid_get_value;
 113         lttng_context_update(*ctx);
 114         return 0;
 115 }
 116
 117 /*
 118  * Force a read (imply TLS fixup for dlopen) of TLS variables.
 119  */
 120 void lttng_fixup_vtid_tls(void)
 121 {
 122         asm volatile ("" : : "m" (URCU_TLS(cached_vtid)));
 123 }