Current trace-cmd requires 'current_tracer' file for an instance, and if
the file is nothing, trace-cmd dies. However, current_tracer file was
introduced from 607e2ea167e patch in Linux kernel 3.14-rc3, so current
trace-cmd cannot use multiple buffers for old kernels.
This patch avoids to die if current_tracer file is nothing in an instance
and the plugin name is "nop". This is because old kernels always trace
as nop mode in an instance.
<How to test>
1) # ./trace-cmd record -B foo -e sched
Before introducing this patch, following error message is output in an old
kernel:
trace-cmd: Permission denied
writing to '/sys/kernel/debug/tracing/instances/foo/current_tracer'
trace-cmd dies in disable_trace() calling set_plugin("nop").
After introducing this patch, trace-cmd works fine in both old kernels and
current kernel.
2) # ./trace-cmd record -B foo -e sched -p nop
It also works fine after introducing this patch in both old kernels and current
kernel.
3) # ./trace-cmd record -B foo -e sched -p function
For old kernels, it dies because an user should not set plugin tracer.
For current kernel, it works fine.
Note that this patch was tested for kernel 3.11.9-200 and 3.15.0-rc2+.
Signed-off-by: Yoshihiro YUNOMAE <[email protected]>
Cc: Steven Rostedt <[email protected]>
---
trace-record.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)
diff --git a/trace-record.c b/trace-record.c
index d3dd0c5..1912175 100644
--- a/trace-record.c
+++ b/trace-record.c
@@ -890,8 +890,18 @@ set_plugin_instance(struct buffer_instance *instance,
const char *name)
path = get_instance_file(instance, "current_tracer");
fp = fopen(path, "w");
- if (!fp)
+ if (!fp) {
+ /*
+ * Legacy kernels do not have current_tracer file, and they
+ * always use nop. So, it doesn't need to try to change the
+ * plugin for those if name is "nop".
+ */
+ if (!strncmp(name, "nop", 3)) {
+ tracecmd_put_tracing_file(path);
+ return;
+ }
die("writing to '%s'", path);
+ }
tracecmd_put_tracing_file(path);
fwrite(name, 1, strlen(name), fp);
--
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