[PATCH v1 7/9] ring-buffer: Avoid reading buffer::subbuf_order on the write path

[Vincent Donnefort <vdonnefort@google.com>]

buffer::subbuf_order is protected by buffer::mutex, which cannot be
held on the write path. While reading it is safe because writers are
disabled during order changes, reading the order from commit_page (which
is pinned for the duration of the commit) avoids confusing context
analysis.

Because rb_subbuf_size() and rb_subbuf_capacity() are no longer on the
write hot path, drop their __always_inline annotations.

Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
---
 kernel/trace/ring_buffer.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 02d3cc193d1e..82f6e3df4627 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -606,7 +606,7 @@ struct trace_buffer {
 	unsigned int			subbuf_order;
 };
 
-static __always_inline unsigned int rb_subbuf_size(struct trace_buffer *buffer)
+static unsigned int rb_subbuf_size(struct trace_buffer *buffer)
 {
 	return PAGE_SIZE << buffer->subbuf_order;
 }
@@ -620,7 +620,7 @@ static __always_inline unsigned int rb_subbuf_size(struct trace_buffer *buffer)
  *
  * Return: The maximum size available for events in a trace buffer subbuffer.
  */
-static __always_inline unsigned int rb_subbuf_capacity(struct trace_buffer *buffer)
+static unsigned int rb_subbuf_capacity(struct trace_buffer *buffer)
 {
 	return rb_subbuf_size(buffer) - BUF_PAGE_HDR_SIZE;
 }
@@ -632,15 +632,15 @@ static unsigned long rb_calculate_nr_pages(struct trace_buffer *buffer, unsigned
 
 /**
  * rb_subbuf_start - Get the start address of a subbuffer
- * @buffer:	A trace buffer
+ * @cpu_buffer:	A per-CPU ring buffer
  * @addr:	An address of an event on a subbuffer
  *
  * Return: The start of the subbuffer for where @addr sits
  */
 static __always_inline
-unsigned long rb_subbuf_start(struct trace_buffer *buffer, unsigned long addr)
+unsigned long rb_subbuf_start(struct ring_buffer_per_cpu *cpu_buffer, unsigned long addr)
 {
-	return addr & ~((unsigned long)(rb_subbuf_size(buffer) - 1));
+	return addr & ~((unsigned long)(PAGE_SIZE << cpu_buffer->commit_page->order) - 1);
 }
 
 static bool rb_is_static(struct ring_buffer_per_cpu *cpu_buffer)
@@ -3582,7 +3582,7 @@ rb_event_index(struct ring_buffer_per_cpu *cpu_buffer, struct ring_buffer_event
 {
 	unsigned long addr = (unsigned long)event;
 
-	addr &= (unsigned long)rb_subbuf_size(cpu_buffer->buffer) - 1;
+	addr &= (unsigned long)(PAGE_SIZE << cpu_buffer->commit_page->order) - 1;
 
 	return addr - BUF_PAGE_HDR_SIZE;
 }
@@ -4168,7 +4168,7 @@ rb_try_to_discard(struct ring_buffer_per_cpu *cpu_buffer,
 
 	new_index = rb_event_index(cpu_buffer, event);
 	old_index = new_index + rb_event_ts_length(event);
-	addr = rb_subbuf_start(cpu_buffer->buffer, (unsigned long)event);
+	addr = rb_subbuf_start(cpu_buffer, (unsigned long)event);
 
 	bpage = READ_ONCE(cpu_buffer->tail_page);
 
@@ -5091,7 +5091,7 @@ rb_decrement_entry(struct ring_buffer_per_cpu *cpu_buffer,
 	struct buffer_page *bpage = cpu_buffer->commit_page;
 	struct buffer_page *start;
 
-	addr = rb_subbuf_start(cpu_buffer->buffer, addr);
+	addr = rb_subbuf_start(cpu_buffer, addr);
 
 	/* Do the likely case first */
 	if (likely(bpage->page == (void *)addr)) {
-- 
2.56.0.567.gdb80ae8cd5-goog