src/lttng-context-preemptible.c

   1 /* SPDX-License-Identifier: (GPL-2.0-only or LGPL-2.1-only)
   2  *
   3  * lttng-context-preemptible.c
   4  *
   5  * LTTng preemptible context.
   6  *
   7  * Copyright (C) 2009-2015 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   8  */
   9
  10 #include <linux/module.h>
  11 #include <linux/slab.h>
  12 #include <linux/sched.h>
  13 #include <linux/irqflags.h>
  14 #include <lttng/events.h>
  15 #include <lttng/events-internal.h>
  16 #include <ringbuffer/frontend_types.h>
  17 #include <wrapper/vmalloc.h>
  18 #include <lttng/tracer.h>
  19
  20 /*
  21  * We nest twice in preempt disabling within LTTng: one nesting is done
  22  * by the instrumentation (tracepoint, kprobes, kretprobes, syscall
  23  * tracepoint), and the second is within the lib ring buffer
  24  * lib_ring_buffer_get_cpu().
  25  */
  26 #define LTTNG_PREEMPT_DISABLE_NESTING   2
  27
  28 static
  29 size_t preemptible_get_size(void *priv, struct lttng_kernel_probe_ctx *probe_ctx, size_t offset)
  30 {
  31         size_t size = 0;
  32
  33         size += lib_ring_buffer_align(offset, lttng_alignof(uint8_t));
  34         size += sizeof(uint8_t);
  35         return size;
  36 }
  37
  38 static
  39 void preemptible_record(void *priv, struct lttng_kernel_probe_ctx *probe_ctx,
  40                 struct lttng_kernel_ring_buffer_ctx *ctx,
  41                 struct lttng_kernel_channel_buffer *chan)
  42 {
  43         int pc = preempt_count();
  44         uint8_t preemptible = 0;
  45
  46         WARN_ON_ONCE(pc < LTTNG_PREEMPT_DISABLE_NESTING);
  47         if (pc == LTTNG_PREEMPT_DISABLE_NESTING)
  48                 preemptible = 1;
  49         chan->ops->event_write(ctx, &preemptible, sizeof(preemptible), lttng_alignof(preemptible));
  50 }
  51
  52 static
  53 void preemptible_get_value(void *priv,
  54                 struct lttng_kernel_probe_ctx *lttng_probe_ctx,
  55                 struct lttng_ctx_value *value)
  56 {
  57         int pc = preempt_count();
  58
  59         WARN_ON_ONCE(pc < LTTNG_PREEMPT_DISABLE_NESTING);
  60         if (pc == LTTNG_PREEMPT_DISABLE_NESTING)
  61                 value->u.s64 = 1;
  62         else
  63                 value->u.s64 = 0;
  64 }
  65
  66 static const struct lttng_kernel_ctx_field *ctx_field = lttng_kernel_static_ctx_field(
  67         lttng_kernel_static_event_field("preemptible",
  68                 lttng_kernel_static_type_integer_from_type(uint8_t, __BYTE_ORDER, 10),
  69                 false, false, false),
  70         preemptible_get_size,
  71         preemptible_record,
  72         preemptible_get_value,
  73         NULL, NULL);
  74
  75 int lttng_add_preemptible_to_ctx(struct lttng_kernel_ctx **ctx)
  76 {
  77         int ret;
  78
  79         if (lttng_kernel_find_context(*ctx, ctx_field->event_field->name))
  80                 return -EEXIST;
  81         ret = lttng_kernel_context_append(ctx, ctx_field);
  82         wrapper_vmalloc_sync_mappings();
  83         return ret;
  84 }
  85 EXPORT_SYMBOL_GPL(lttng_add_preemptible_to_ctx);