On Tue, 11 Jun 2024 15:43:59 -0700
Guenter Roeck [off-list ref] wrote:

On 6/11/24 12:28, Steven Rostedt wrote:

quoted

From: "Steven Rostedt (Google)" <rostedt@goodmis.org>

In preparation for having the ring buffer mapped to a dedicated location,
which will have the same restrictions as user space memory mapped buffers,
allow it to use the "mapped" field of the ring_buffer_per_cpu structure
without having the user space meta page mapping.

When this starts using the mapped field, it will need to handle adding a
user space mapping (and removing it) from a ring buffer that is using a
dedicated memory range.

Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
  kernel/trace/ring_buffer.c | 11 ++++++++---
  1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 28853966aa9a..78beaccf9c8c 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c

@@ -5224,6 +5224,9 @@ static void rb_update_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
  {
  	struct trace_buffer_meta *meta = cpu_buffer->meta_page;
  
+	if (!meta)
+		return;
+
  	meta->reader.read = cpu_buffer->reader_page->read;
  	meta->reader.id = cpu_buffer->reader_page->id;
  	meta->reader.lost_events = cpu_buffer->lost_events;

@@ -6167,7 +6170,7 @@ rb_get_mapped_buffer(struct trace_buffer *buffer, int cpu)
  
  	mutex_lock(&cpu_buffer->mapping_lock);
  
-	if (!cpu_buffer->mapped) {
+	if (!cpu_buffer->mapped || !cpu_buffer->meta_page) {
  		mutex_unlock(&cpu_buffer->mapping_lock);
  		return ERR_PTR(-ENODEV);
  	}

@@ -6359,12 +6362,13 @@ int ring_buffer_map(struct trace_buffer *buffer, int cpu,
  	 */
  	raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
  	rb_setup_ids_meta_page(cpu_buffer, subbuf_ids);
+  

Picky again. Is that a leftover from something ? I don't see an immediate reason
for the added newline.

Hmm, I could remove it.

quoted

  	raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
  
  	err = __rb_map_vma(cpu_buffer, vma);
  	if (!err) {
  		raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
-		cpu_buffer->mapped = 1;
+		cpu_buffer->mapped++;
  		raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
  	} else {
  		kfree(cpu_buffer->subbuf_ids);

@@ -6403,7 +6407,8 @@ int ring_buffer_unmap(struct trace_buffer *buffer, int cpu)
  	mutex_lock(&buffer->mutex);
  	raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
  
-	cpu_buffer->mapped = 0;
+	WARN_ON_ONCE(!cpu_buffer->mapped);
+	cpu_buffer->mapped--;  

This will wrap to UINT_MAX if it was 0. Is that intentional ?

If mapped is non zero, it limits what it can do. If it enters here as zero,
we are really in a unknown state, so yeah, wrapping will just keep it
limited. Which is a good thing.

Do you want me to add a comment there?

-- Steve

quoted

  
  	raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);