From: "Steven Rostedt (Google)" <rostedt@goodmis.org>
Date: Sat, 15 Jul 2023 10:12:14 -0400
Subject: tracing: Remove unnecessary copying of tr->current_trace
Git-commit: 9182b519b8c9733e921165f93e63cfffc18d1e0a
Patch-mainline: v6.6-rc1
References: git-fixes

The iterator allocated a descriptor to copy the current_trace. This was done
with the assumption that the function pointers might change. But this was a
false assuption, as it does not change. There's no reason to make a copy of the
current_trace and just use the pointer it points to. This removes needing to
manage freeing the descriptor. Worse yet, there's locations that the iterator
is used but does make a copy and just uses the pointer. This could cause the
actual pointer to the trace descriptor to be freed and not the allocated copy.

This is more of a clean up than a fix.

Link: https://lkml.kernel.org/r/20230715141348.135792275@goodmis.org

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Fixes: d7350c3f45694 ("tracing/core: make the read callbacks reentrants")
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Acked-by: Petr Pavlu <petr.pavlu@suse.com>
---
 kernel/trace/trace.c | 22 +++-------------------
 1 file changed, 3 insertions(+), 19 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index f6ed6b79d91f..5e665c356df7 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -4189,18 +4189,12 @@ static void *s_start(struct seq_file *m, loff_t *pos)
 	loff_t l = 0;
 	int cpu;
 
-	/*
-	 * 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(tr->current_trace && iter->trace->name != tr->current_trace->name)) {
+	if (unlikely(tr->current_trace != iter->trace)) {
 		/* Close iter->trace before switching to the new current tracer */
 		if (iter->trace->close)
 			iter->trace->close(iter);
-		*iter->trace = *tr->current_trace;
+		iter->trace = tr->current_trace;
 		/* Reopen the new current tracer */
 		if (iter->trace->open)
 			iter->trace->open(iter);
@@ -4846,16 +4840,8 @@ __tracing_open(struct inode *inode, struct file *file, bool snapshot)
 	iter->fmt = NULL;
 	iter->fmt_size = 0;
 
-	/*
-	 * We make a copy of the current tracer to avoid concurrent
-	 * changes on it while we are reading.
-	 */
 	mutex_lock(&trace_types_lock);
-	iter->trace = kzalloc(sizeof(*iter->trace), GFP_KERNEL);
-	if (!iter->trace)
-		goto fail;
-
-	*iter->trace = *tr->current_trace;
+	iter->trace = tr->current_trace;
 
 	if (!zalloc_cpumask_var(&iter->started, GFP_KERNEL))
 		goto fail;
@@ -4920,7 +4906,6 @@ __tracing_open(struct inode *inode, struct file *file, bool snapshot)
 
  fail:
 	mutex_unlock(&trace_types_lock);
-	kfree(iter->trace);
 	kfree(iter->temp);
 	kfree(iter->buffer_iter);
 release:
@@ -5005,7 +4990,6 @@ static int tracing_release(struct inode *inode, struct file *file)
 	free_cpumask_var(iter->started);
 	kfree(iter->fmt);
 	kfree(iter->temp);
-	kfree(iter->trace);
 	kfree(iter->buffer_iter);
 	seq_release_private(inode, file);