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 <ust-tid.h>
  17 #include <urcu/tls-compat.h>
  18
  19 #include "context-internal.h"
  20 #include "lttng-tracer-core.h"
  21
  22 /*
  23  * We cache the result to ensure we don't trigger a system call for
  24  * each event.
  25  */
  26 static DEFINE_URCU_TLS(pid_t, cached_vtid);
  27
  28 /*
  29  * Upon fork or clone, the TID assigned to our thread is not the same as
  30  * we kept in cache. Luckily, we are the only thread surviving in the
  31  * child process, so we can simply clear our cached version.
  32  */
  33 void lttng_context_vtid_reset(void)
  34 {
  35         CMM_STORE_SHARED(URCU_TLS(cached_vtid), 0);
  36 }
  37
  38 static
  39 size_t vtid_get_size(struct lttng_ust_ctx_field *field, size_t offset)
  40 {
  41         size_t size = 0;
  42
  43         size += lib_ring_buffer_align(offset, lttng_alignof(pid_t));
  44         size += sizeof(pid_t);
  45         return size;
  46 }
  47
  48 static inline
  49 pid_t wrapper_getvtid(void)
  50 {
  51         pid_t vtid;
  52
  53         vtid = CMM_LOAD_SHARED(URCU_TLS(cached_vtid));
  54         if (caa_unlikely(!vtid)) {
  55                 vtid = lttng_gettid();
  56                 CMM_STORE_SHARED(URCU_TLS(cached_vtid), vtid);
  57         }
  58         return vtid;
  59 }
  60
  61 static
  62 void vtid_record(struct lttng_ust_ctx_field *field,
  63                  struct lttng_ust_lib_ring_buffer_ctx *ctx,
  64                  struct lttng_channel *chan)
  65 {
  66         pid_t vtid = wrapper_getvtid();
  67
  68         lib_ring_buffer_align_ctx(ctx, lttng_alignof(vtid));
  69         chan->ops->event_write(ctx, &vtid, sizeof(vtid));
  70 }
  71
  72 static
  73 void vtid_get_value(struct lttng_ust_ctx_field *field,
  74                 struct lttng_ust_ctx_value *value)
  75 {
  76         value->u.s64 = wrapper_getvtid();
  77 }
  78
  79 int lttng_add_vtid_to_ctx(struct lttng_ust_ctx **ctx)
  80 {
  81         struct lttng_ust_ctx_field *field;
  82
  83         field = lttng_append_context(ctx);
  84         if (!field)
  85                 return -ENOMEM;
  86         if (lttng_find_context(*ctx, "vtid")) {
  87                 lttng_remove_context_field(ctx, field);
  88                 return -EEXIST;
  89         }
  90         field->event_field->name = "vtid";
  91         field->event_field->type.atype = atype_integer;
  92         field->event_field->type.u.integer.size = sizeof(pid_t) * CHAR_BIT;
  93         field->event_field->type.u.integer.alignment = lttng_alignof(pid_t) * CHAR_BIT;
  94         field->event_field->type.u.integer.signedness = lttng_is_signed_type(pid_t);
  95         field->event_field->type.u.integer.reverse_byte_order = 0;
  96         field->event_field->type.u.integer.base = 10;
  97         field->event_field->type.u.integer.encoding = lttng_encode_none;
  98         field->get_size = vtid_get_size;
  99         field->record = vtid_record;
 100         field->get_value = vtid_get_value;
 101         lttng_context_update(*ctx);
 102         return 0;
 103 }
 104
 105 /*
 106  * Force a read (imply TLS fixup for dlopen) of TLS variables.
 107  */
 108 void lttng_fixup_vtid_tls(void)
 109 {
 110         asm volatile ("" : : "m" (URCU_TLS(cached_vtid)));
 111 }