ring_buffer: tracing: Inherit the tracing setting to next ring buffer
authorMasami Hiramatsu <mhiramat@kernel.org>
Fri, 13 Jul 2018 16:28:15 +0000 (01:28 +0900)
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Thu, 9 Aug 2018 10:16:38 +0000 (12:16 +0200)
commit60baabc37bc18323e6346d28fd7b70729e345703
treec36cadf861472763a243a7e6942fd60e20ecc669
parentff28e5cc58c2015ae65ad0070a3c90898f69aa3b
ring_buffer: tracing: Inherit the tracing setting to next ring buffer

commit 73c8d8945505acdcbae137c2e00a1232e0be709f upstream.

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: debdd57f5145 ("tracing: Make a snapshot feature available from userspace")
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
[ Updated commit log and comment in the code ]
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
include/linux/ring_buffer.h
kernel/trace/ring_buffer.c
kernel/trace/trace.c