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 <sys/types.h>
  25 #include <unistd.h>
  26 #include <lttng/ust-events.h>
  27 #include <lttng/ust-tracer.h>
  28 #include <lttng/ringbuffer-config.h>
  29 #include <lttng/ust-tid.h>
  30 #include <urcu/tls-compat.h>
  31 #include "lttng-tracer-core.h"
  32
  33 /*
  34  * We cache the result to ensure we don't trigger a system call for
  35  * each event.
  36  */
  37 static DEFINE_URCU_TLS(pid_t, cached_vtid);
  38
  39 /*
  40  * Upon fork or clone, the TID assigned to our thread is not the same as
  41  * we kept in cache. Luckily, we are the only thread surviving in the
  42  * child process, so we can simply clear our cached version.
  43  */
  44 void lttng_context_vtid_reset(void)
  45 {
  46         CMM_STORE_SHARED(URCU_TLS(cached_vtid), 0);
  47 }
  48
  49 static
  50 size_t vtid_get_size(struct lttng_ctx_field *field, size_t offset)
  51 {
  52         size_t size = 0;
  53
  54         size += lib_ring_buffer_align(offset, lttng_alignof(pid_t));
  55         size += sizeof(pid_t);
  56         return size;
  57 }
  58
  59 static inline
  60 pid_t wrapper_getvtid(void)
  61 {
  62         pid_t vtid;
  63
  64         vtid = CMM_LOAD_SHARED(URCU_TLS(cached_vtid));
  65         if (caa_unlikely(!vtid)) {
  66                 vtid = lttng_gettid();
  67                 CMM_STORE_SHARED(URCU_TLS(cached_vtid), vtid);
  68         }
  69         return vtid;
  70 }
  71
  72 static
  73 void vtid_record(struct lttng_ctx_field *field,
  74                  struct lttng_ust_lib_ring_buffer_ctx *ctx,
  75                  struct lttng_channel *chan)
  76 {
  77         pid_t vtid = wrapper_getvtid();
  78
  79         lib_ring_buffer_align_ctx(ctx, lttng_alignof(vtid));
  80         chan->ops->event_write(ctx, &vtid, sizeof(vtid));
  81 }
  82
  83 static
  84 void vtid_get_value(struct lttng_ctx_field *field,
  85                 struct lttng_ctx_value *value)
  86 {
  87         value->u.s64 = wrapper_getvtid();
  88 }
  89
  90 int lttng_add_vtid_to_ctx(struct lttng_ctx **ctx)
  91 {
  92         struct lttng_ctx_field *field;
  93
  94         field = lttng_append_context(ctx);
  95         if (!field)
  96                 return -ENOMEM;
  97         if (lttng_find_context(*ctx, "vtid")) {
  98                 lttng_remove_context_field(ctx, field);
  99                 return -EEXIST;
 100         }
 101         field->event_field.name = "vtid";
 102         field->event_field.type.atype = atype_integer;
 103         field->event_field.type.u.basic.integer.size = sizeof(pid_t) * CHAR_BIT;
 104         field->event_field.type.u.basic.integer.alignment = lttng_alignof(pid_t) * CHAR_BIT;
 105         field->event_field.type.u.basic.integer.signedness = lttng_is_signed_type(pid_t);
 106         field->event_field.type.u.basic.integer.reverse_byte_order = 0;
 107         field->event_field.type.u.basic.integer.base = 10;
 108         field->event_field.type.u.basic.integer.encoding = lttng_encode_none;
 109         field->get_size = vtid_get_size;
 110         field->record = vtid_record;
 111         field->get_value = vtid_get_value;
 112         lttng_context_update(*ctx);
 113         return 0;
 114 }
 115
 116 /*
 117  * Force a read (imply TLS fixup for dlopen) of TLS variables.
 118  */
 119 void lttng_fixup_vtid_tls(void)
 120 {
 121         asm volatile ("" : : "m" (URCU_TLS(cached_vtid)));
 122 }