• Steven Rostedt's avatar
    tracing: Disable buffer switching when starting or stopping trace · a2f80714
    Steven Rostedt authored
    When the trace iterator is read, tracing_start() and tracing_stop()
    is called to stop tracing while the iterator is processing the trace
    output.
    
    These functions disable both the standard buffer and the max latency
    buffer. But if the wakeup tracer is running, it can switch these
    buffers between the two disables:
    
      buffer = global_trace.buffer;
      if (buffer)
          ring_buffer_record_disable(buffer);
    
          <<<--------- swap happens here
    
      buffer = max_tr.buffer;
      if (buffer)
          ring_buffer_record_disable(buffer);
    
    What happens is that we disabled the same buffer twice. On tracing_start()
    we can enable the same buffer twice. All ring_buffer_record_disable()
    must be matched with a ring_buffer_record_enable() or the buffer
    can be disable permanently, or enable prematurely, and cause a bug
    where a reset happens while a trace is commiting.
    
    This patch protects these two by taking the ftrace_max_lock to prevent
    a switch from occurring.
    
    Found with Li Zefan's ftrace_stress_test.
    
    Cc: stable@kernel.org
    Reported-by: default avatarLai Jiangshan <laijs@cn.fujitsu.com>
    Signed-off-by: default avatarSteven Rostedt <rostedt@goodmis.org>
    a2f80714
trace.c 102 KB