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[PATCH v2] tracing: Choose static tp_printk buffer by explicit nesting count

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[PATCH v2] tracing: Choose static tp_printk buffer by explicit nesting count

Andy Lutomirski-3
Currently, the trace_printk code chooses which static buffer to use based
on what type of atomic context (NMI, IRQ, etc) it's in.  Simplify the
code and make it more robust: simply count the nesting depth and choose
a buffer based on the current nesting depth.

The new code will only drop an event if we nest more than 4 deep,
and the old code was guaranteed to malfunction if that happened.

Signed-off-by: Andy Lutomirski <[hidden email]>
---

Changes from v1: Fix error handling (Namhyung Kim)

kernel/trace/trace.c | 83 +++++++++++++++-------------------------------------
 1 file changed, 24 insertions(+), 59 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index a2f0b9f33e9b..b3d703bda0a9 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -1986,83 +1986,41 @@ static void __trace_userstack(struct trace_array *tr, unsigned long flags)
 
 /* created for use with alloc_percpu */
 struct trace_buffer_struct {
- char buffer[TRACE_BUF_SIZE];
+ int nesting;
+ char buffer[4][TRACE_BUF_SIZE];
 };
 
 static struct trace_buffer_struct *trace_percpu_buffer;
-static struct trace_buffer_struct *trace_percpu_sirq_buffer;
-static struct trace_buffer_struct *trace_percpu_irq_buffer;
-static struct trace_buffer_struct *trace_percpu_nmi_buffer;
 
 /*
- * The buffer used is dependent on the context. There is a per cpu
- * buffer for normal context, softirq contex, hard irq context and
- * for NMI context. Thise allows for lockless recording.
- *
- * Note, if the buffers failed to be allocated, then this returns NULL
+ * Thise allows for lockless recording.  If we're nested too deeply, then
+ * this returns NULL.
  */
 static char *get_trace_buf(void)
 {
- struct trace_buffer_struct *percpu_buffer;
-
- /*
- * If we have allocated per cpu buffers, then we do not
- * need to do any locking.
- */
- if (in_nmi())
- percpu_buffer = trace_percpu_nmi_buffer;
- else if (in_irq())
- percpu_buffer = trace_percpu_irq_buffer;
- else if (in_softirq())
- percpu_buffer = trace_percpu_sirq_buffer;
- else
- percpu_buffer = trace_percpu_buffer;
+ struct trace_buffer_struct *buffer = this_cpu_ptr(trace_percpu_buffer);
 
- if (!percpu_buffer)
+ if (!buffer || buffer->nesting >= 4)
  return NULL;
 
- return this_cpu_ptr(&percpu_buffer->buffer[0]);
+ return &buffer->buffer[buffer->nesting++][0];
+}
+
+static void put_trace_buf(void)
+{
+ this_cpu_dec(trace_percpu_buffer->nesting);
 }
 
 static int alloc_percpu_trace_buffer(void)
 {
  struct trace_buffer_struct *buffers;
- struct trace_buffer_struct *sirq_buffers;
- struct trace_buffer_struct *irq_buffers;
- struct trace_buffer_struct *nmi_buffers;
 
  buffers = alloc_percpu(struct trace_buffer_struct);
- if (!buffers)
- goto err_warn;
-
- sirq_buffers = alloc_percpu(struct trace_buffer_struct);
- if (!sirq_buffers)
- goto err_sirq;
-
- irq_buffers = alloc_percpu(struct trace_buffer_struct);
- if (!irq_buffers)
- goto err_irq;
-
- nmi_buffers = alloc_percpu(struct trace_buffer_struct);
- if (!nmi_buffers)
- goto err_nmi;
+ if (WARN(!buffers, "Could not allocate percpu trace_printk buffer"))
+ return -ENOMEM;
 
  trace_percpu_buffer = buffers;
- trace_percpu_sirq_buffer = sirq_buffers;
- trace_percpu_irq_buffer = irq_buffers;
- trace_percpu_nmi_buffer = nmi_buffers;
-
  return 0;
-
- err_nmi:
- free_percpu(irq_buffers);
- err_irq:
- free_percpu(sirq_buffers);
- err_sirq:
- free_percpu(buffers);
- err_warn:
- WARN(1, "Could not allocate percpu trace_printk buffer");
- return -ENOMEM;
 }
 
 static int buffers_allocated;
@@ -2153,7 +2111,7 @@ int trace_vbprintk(unsigned long ip, const char *fmt, va_list args)
  tbuffer = get_trace_buf();
  if (!tbuffer) {
  len = 0;
- goto out;
+ goto out_nobuffer;
  }
 
  len = vbin_printf((u32 *)tbuffer, TRACE_BUF_SIZE/sizeof(int), fmt, args);
@@ -2179,6 +2137,9 @@ int trace_vbprintk(unsigned long ip, const char *fmt, va_list args)
  }
 
 out:
+ put_trace_buf();
+
+out_nobuffer:
  preempt_enable_notrace();
  unpause_graph_tracing();
 
@@ -2210,7 +2171,7 @@ __trace_array_vprintk(struct ring_buffer *buffer,
  tbuffer = get_trace_buf();
  if (!tbuffer) {
  len = 0;
- goto out;
+ goto out_nobuffer;
  }
 
  len = vscnprintf(tbuffer, TRACE_BUF_SIZE, fmt, args);
@@ -2229,7 +2190,11 @@ __trace_array_vprintk(struct ring_buffer *buffer,
  __buffer_unlock_commit(buffer, event);
  ftrace_trace_stack(&global_trace, buffer, flags, 6, pc, NULL);
  }
- out:
+
+out:
+ put_trace_buf();
+
+out_nobuffer:
  preempt_enable_notrace();
  unpause_graph_tracing();
 
--
2.5.5

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