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ring_buffer: tracing: Inherit the tracing setting to next ring buffer 

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: debdd57f51 ("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>
This commit is contained in:
Masami Hiramatsu 2018-07-14 01:28:15 +09:00 committed by Steven Rostedt (VMware)
parent 1863c38725
commit 73c8d89455
3 changed files with 23 additions and 0 deletions
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1
include/linux/ring_buffer.h
View File

@ -165,6 +165,7 @@ void ring_buffer_record_enable(struct ring_buffer *buffer);
void ring_buffer_record_off(struct ring_buffer *buffer);
void ring_buffer_record_on(struct ring_buffer *buffer);
int ring_buffer_record_is_on(struct ring_buffer *buffer);
int ring_buffer_record_is_set_on(struct ring_buffer *buffer);
void ring_buffer_record_disable_cpu(struct ring_buffer *buffer, int cpu);
void ring_buffer_record_enable_cpu(struct ring_buffer *buffer, int cpu);

16
kernel/trace/ring_buffer.c
View File

@ -3226,6 +3226,22 @@ int ring_buffer_record_is_on(struct ring_buffer *buffer)
return !atomic_read(&buffer->record_disabled);
}
/**
* ring_buffer_record_is_set_on - return true if the ring buffer is set writable
* @buffer: The ring buffer to see if write is set enabled
*
* Returns true if the ring buffer is set writable by ring_buffer_record_on().
* Note that this does NOT mean it is in a writable state.
*
* It may return true when the ring buffer has been disabled by
* ring_buffer_record_disable(), as that is a temporary disabling of
* the ring buffer.
*/
int ring_buffer_record_is_set_on(struct ring_buffer *buffer)
{
return !(atomic_read(&buffer->record_disabled) & RB_BUFFER_OFF);
}
/**
* ring_buffer_record_disable_cpu - stop all writes into the cpu_buffer
* @buffer: The ring buffer to stop writes to.

6
kernel/trace/trace.c
View File

@ -1373,6 +1373,12 @@ update_max_tr(struct trace_array *tr, struct task_struct *tsk, int cpu)
arch_spin_lock(&tr->max_lock);
/* Inherit the recordable setting from trace_buffer */
if (ring_buffer_record_is_set_on(tr->trace_buffer.buffer))
ring_buffer_record_on(tr->max_buffer.buffer);
else
ring_buffer_record_off(tr->max_buffer.buffer);
swap(tr->trace_buffer.buffer, tr->max_buffer.buffer);
__update_max_tr(tr, tsk, cpu);