• Steven Rostedt (VMware)'s avatar
    fgraph: Initialize tracing_graph_pause at task creation · 7e0a9220
    Steven Rostedt (VMware) authored
    On some archs, the idle task can call into cpu_suspend(). The cpu_suspend()
    will disable or pause function graph tracing, as there's some paths in
    bringing down the CPU that can have issues with its return address being
    modified. The task_struct structure has a "tracing_graph_pause" atomic
    counter, that when set to something other than zero, the function graph
    tracer will not modify the return address.
    
    The problem is that the tracing_graph_pause counter is initialized when the
    function graph tracer is enabled. This can corrupt the counter for the idle
    task if it is suspended in these architectures.
    
       CPU 1				CPU 2
       -----				-----
      do_idle()
        cpu_suspend()
          pause_graph_tracing()
              task_struct->tracing_graph_pause++ (0 -> 1)
    
    				start_graph_tracing()
    				  for_each_online_cpu(cpu) {
    				    ftrace_graph_init_idle_task(cpu)
    				      task-struct->tracing_graph_pause = 0 (1 -> 0)
    
          unpause_graph_tracing()
              task_struct->tracing_graph_pause-- (0 -> -1)
    
    The above should have gone from 1 to zero, and enabled function graph
    tracing again. But instead, it is set to -1, which keeps it disabled.
    
    There's no reason that the field tracing_graph_pause on the task_struct can
    not be initialized at boot up.
    
    Cc: stable@vger.kernel.org
    Fixes: 380c4b14 ("tracing/function-graph-tracer: append the tracing_graph_flag")
    Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=211339
    Reported-by: pierre.gondois@arm.com
    Signed-off-by: default avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
    7e0a9220
fgraph.c 16.8 KB