liblttng-ust/lttng-context-vtid.c

   1 /*
   2  * SPDX-License-Identifier: LGPL-2.1-only
   3  *
   4  * Copyright (C) 2009-2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   5  *
   6  * LTTng UST vtid context.
   7  */
   8
   9 #define _LGPL_SOURCE
  10 #include <stddef.h>
  11 #include <sys/types.h>
  12 #include <unistd.h>
  13 #include <lttng/ust-events.h>
  14 #include <lttng/ust-tracer.h>
  15 #include <lttng/ringbuffer-config.h>
  16 #include <lttng/ust-tid.h>
  17 #include <urcu/tls-compat.h>
  18 #include "lttng-tracer-core.h"
  19
  20 /*
  21  * We cache the result to ensure we don't trigger a system call for
  22  * each event.
  23  */
  24 static DEFINE_URCU_TLS(pid_t, cached_vtid);
  25
  26 /*
  27  * Upon fork or clone, the TID assigned to our thread is not the same as
  28  * we kept in cache. Luckily, we are the only thread surviving in the
  29  * child process, so we can simply clear our cached version.
  30  */
  31 void lttng_context_vtid_reset(void)
  32 {
  33         CMM_STORE_SHARED(URCU_TLS(cached_vtid), 0);
  34 }
  35
  36 static
  37 size_t vtid_get_size(struct lttng_ctx_field *field, size_t offset)
  38 {
  39         size_t size = 0;
  40
  41         size += lib_ring_buffer_align(offset, lttng_alignof(pid_t));
  42         size += sizeof(pid_t);
  43         return size;
  44 }
  45
  46 static inline
  47 pid_t wrapper_getvtid(void)
  48 {
  49         pid_t vtid;
  50
  51         vtid = CMM_LOAD_SHARED(URCU_TLS(cached_vtid));
  52         if (caa_unlikely(!vtid)) {
  53                 vtid = lttng_gettid();
  54                 CMM_STORE_SHARED(URCU_TLS(cached_vtid), vtid);
  55         }
  56         return vtid;
  57 }
  58
  59 static
  60 void vtid_record(struct lttng_ctx_field *field,
  61                  struct lttng_ust_lib_ring_buffer_ctx *ctx,
  62                  struct lttng_channel *chan)
  63 {
  64         pid_t vtid = wrapper_getvtid();
  65
  66         lib_ring_buffer_align_ctx(ctx, lttng_alignof(vtid));
  67         chan->ops->event_write(ctx, &vtid, sizeof(vtid));
  68 }
  69
  70 static
  71 void vtid_get_value(struct lttng_ctx_field *field,
  72                 struct lttng_ctx_value *value)
  73 {
  74         value->u.s64 = wrapper_getvtid();
  75 }
  76
  77 int lttng_add_vtid_to_ctx(struct lttng_ctx **ctx)
  78 {
  79         struct lttng_ctx_field *field;
  80
  81         field = lttng_append_context(ctx);
  82         if (!field)
  83                 return -ENOMEM;
  84         if (lttng_find_context(*ctx, "vtid")) {
  85                 lttng_remove_context_field(ctx, field);
  86                 return -EEXIST;
  87         }
  88         field->event_field.name = "vtid";
  89         field->event_field.type.atype = atype_integer;
  90         field->event_field.type.u.integer.size = sizeof(pid_t) * CHAR_BIT;
  91         field->event_field.type.u.integer.alignment = lttng_alignof(pid_t) * CHAR_BIT;
  92         field->event_field.type.u.integer.signedness = lttng_is_signed_type(pid_t);
  93         field->event_field.type.u.integer.reverse_byte_order = 0;
  94         field->event_field.type.u.integer.base = 10;
  95         field->event_field.type.u.integer.encoding = lttng_encode_none;
  96         field->get_size = vtid_get_size;
  97         field->record = vtid_record;
  98         field->get_value = vtid_get_value;
  99         lttng_context_update(*ctx);
 100         return 0;
 101 }
 102
 103 /*
 104  * Force a read (imply TLS fixup for dlopen) of TLS variables.
 105  */
 106 void lttng_fixup_vtid_tls(void)
 107 {
 108         asm volatile ("" : : "m" (URCU_TLS(cached_vtid)));
 109 }