tracing: Replace static old_tracer check of tracer name

Currently the trace buffer read functions use a static variable
"old_tracer" for detecting if the current tracer changes. This
was suitable for a single trace file ("trace"), but to add a
snapshot feature that will use the same function for its file,
a check against a static variable is not sufficient.

To use the output functions for two different files, instead of
storing the current tracer in a static variable, as the trace
iterator descriptor contains a pointer to the original current
tracer's name, that pointer can now be used to check if the
current tracer has changed between different reads of the trace
file.

Link: http://lkml.kernel.org/r/20121226025252.3252.9276.stgit@liselsia

Signed-off-by: Hiraku Toyooka <hiraku.toyooka.gu@hitachi.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 9 insertions, 13 deletions

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 90a1c71fdbfc..2c724662a3e8 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -1948,18 +1948,20 @@ void tracing_iter_reset(struct trace_iterator *iter, int cpu)
 static void *s_start(struct seq_file *m, loff_t *pos)
 {
         struct trace_iterator *iter = m->private;
-        static struct tracer *old_tracer;
         int cpu_file = iter->cpu_file;
         void *p = NULL;
         loff_t l = 0;
         int cpu;
 
-        /* copy the tracer to avoid using a global lock all around */
+        /*
+         * copy the tracer to avoid using a global lock all around.
+         * iter->trace is a copy of current_trace, the pointer to the
+         * name may be used instead of a strcmp(), as iter->trace->name
+         * will point to the same string as current_trace->name.
+         */
         mutex_lock(&trace_types_lock);
-        if (unlikely(old_tracer != current_trace && current_trace)) {
-                old_tracer = current_trace;
+        if (unlikely(current_trace && iter->trace->name != current_trace->name))
                 *iter->trace = *current_trace;
-        }
         mutex_unlock(&trace_types_lock);
 
         atomic_inc(&trace_record_cmdline_disabled);
@@ -3494,7 +3496,6 @@ tracing_read_pipe(struct file *filp, char __user *ubuf,
                   size_t cnt, loff_t *ppos)
 {
         struct trace_iterator *iter = filp->private_data;
-        static struct tracer *old_tracer;
         ssize_t sret;
 
         /* return any leftover data */
@@ -3506,10 +3507,8 @@ tracing_read_pipe(struct file *filp, char __user *ubuf,
 
         /* copy the tracer to avoid using a global lock all around */
         mutex_lock(&trace_types_lock);
-        if (unlikely(old_tracer != current_trace && current_trace)) {
-                old_tracer = current_trace;
+        if (unlikely(current_trace && iter->trace->name != current_trace->name))
                 *iter->trace = *current_trace;
-        }
         mutex_unlock(&trace_types_lock);
 
         /*
@@ -3665,7 +3664,6 @@ static ssize_t tracing_splice_read_pipe(struct file *filp,
                 .ops            = &tracing_pipe_buf_ops,
                 .spd_release    = tracing_spd_release_pipe,
         };
-        static struct tracer *old_tracer;
         ssize_t ret;
         size_t rem;
         unsigned int i;
@@ -3675,10 +3673,8 @@ static ssize_t tracing_splice_read_pipe(struct file *filp,
 
         /* copy the tracer to avoid using a global lock all around */
         mutex_lock(&trace_types_lock);
-        if (unlikely(old_tracer != current_trace && current_trace)) {
-                old_tracer = current_trace;
+        if (unlikely(current_trace && iter->trace->name != current_trace->name))
                 *iter->trace = *current_trace;
-        }
         mutex_unlock(&trace_types_lock);
 
         mutex_lock(&iter->mutex);