• Masami Hiramatsu's avatar
    ring_buffer: tracing: Inherit the tracing setting to next ring buffer · 73c8d894
    Masami Hiramatsu authored
    Maintain the tracing on/off setting of the ring_buffer when switching
    to the trace buffer snapshot.
    
    Taking a snapshot is done by swapping the backup ring buffer
    (max_tr_buffer). But since the tracing on/off setting is defined
    by the ring buffer, when swapping it, the tracing on/off setting
    can also be changed. This causes a strange result like below:
    
      /sys/kernel/debug/tracing # cat tracing_on
      1
      /sys/kernel/debug/tracing # echo 0 > tracing_on
      /sys/kernel/debug/tracing # cat tracing_on
      0
      /sys/kernel/debug/tracing # echo 1 > snapshot
      /sys/kernel/debug/tracing # cat tracing_on
      1
      /sys/kernel/debug/tracing # echo 1 > snapshot
      /sys/kernel/debug/tracing # cat tracing_on
      0
    
    We don't touch tracing_on, but snapshot changes tracing_on
    setting each time. This is an anomaly, because user doesn't know
    that each "ring_buffer" stores its own tracing-enable state and
    the snapshot is done by swapping ring buffers.
    
    Link: http://lkml.kernel.org/r/153149929558.11274.11730609978254724394.stgit@devbox
    
    Cc: Ingo Molnar <mingo@redhat.com>
    Cc: Shuah Khan <shuah@kernel.org>
    Cc: Tom Zanussi <tom.zanussi@linux.intel.com>
    Cc: Hiraku Toyooka <hiraku.toyooka@cybertrust.co.jp>
    Cc: stable@vger.kernel.org
    Fixes: debdd57f ("tracing: Make a snapshot feature available from userspace")
    Signed-off-by: default avatarMasami Hiramatsu <mhiramat@kernel.org>
    [ Updated commit log and comment in the code ]
    Signed-off-by: default avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
    73c8d894
ring_buffer.c 136 KB