[PATCH v2 0/2] ring-buffer: Clean up resize_disabled checks

[PATCH v2 0/2] ring-buffer: Clean up resize_disabled checks

[Vincent Donnefort]

ring_buffer_subbuf_order_set() checks for resize_disabled twice under
the same buffer->mutex hold. Moreover, this check duplicates the logic
in ring_buffer_resize(). Create a common helper rb_resize_disabled() to
factor out this code and drop redundant checks.

Additionally, remove the unnecessary cpumask_test_cpu() in
ring_buffer_subbuf_order_set() as for_each_buffer_cpu() already
iterates over buffer->cpumask.

v2:
  - Split cpumask_test_cpu() cleanup into its own patch (Steven)
  - Remove another unnecessary cpumask_test_cpu() (David)

v1: https://lore.kernel.org/all/20260914175907.1027349-1-vdonnefort@google.com/

Vincent Donnefort (2):
  ring-buffer: Clean up resize_disabled checks
  ring-buffer: Remove unnecessary cpumask_test_cpu

 kernel/trace/ring_buffer.c | 70 +++++++++++++++-----------------------
 1 file changed, 28 insertions(+), 42 deletions(-)


base-commit: fd73f4a6659897191fa0d40695fe370925dd3780
-- 
2.55.0.1082.g2b9226bbc0-goog

[PATCH v2 1/2] ring-buffer: Clean up resize_disabled checks

[Vincent Donnefort]

ring_buffer_subbuf_order_set() checks for resize_disabled twice under
the same buffer->mutex hold. Moreover, this check duplicates the logic
in ring_buffer_resize(). Create a common helper rb_resize_disabled() to
factor out this code.

Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
---
 kernel/trace/ring_buffer.c | 63 +++++++++++++++++---------------------
 1 file changed, 28 insertions(+), 35 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 04bb94c29f58..73311c4fc556 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -3280,6 +3280,21 @@ static void update_pages_handler(struct work_struct *work)
 	complete(&cpu_buffer->update_done);
 }
 
+static bool rb_resize_disabled(struct trace_buffer *buffer, int cpu)
+{
+	lockdep_assert_held(&buffer->mutex);
+
+	if (cpu != RING_BUFFER_ALL_CPUS)
+		return atomic_read(&buffer->buffers[cpu]->resize_disabled);
+
+	for_each_buffer_cpu(buffer, cpu) {
+		if (atomic_read(&buffer->buffers[cpu]->resize_disabled))
+			return true;
+	}
+
+	return false;
+}
+
 /**
  * ring_buffer_resize - resize the ring buffer
  * @buffer: the buffer to resize.
@@ -3324,20 +3339,17 @@ int ring_buffer_resize(struct trace_buffer *buffer, unsigned long size,
 	if (nr_pages < 2)
 		nr_pages = 2;
 
-	if (cpu_id == RING_BUFFER_ALL_CPUS) {
-		/*
-		 * Don't succeed if resizing is disabled, as a reader might be
-		 * manipulating the ring buffer and is expecting a sane state while
-		 * this is true.
-		 */
-		for_each_buffer_cpu(buffer, cpu) {
-			cpu_buffer = buffer->buffers[cpu];
-			if (atomic_read(&cpu_buffer->resize_disabled)) {
-				err = -EBUSY;
-				goto out_err_unlock;
-			}
-		}
+	/*
+	 * Don't succeed if resizing is disabled, as a reader might be
+	 * manipulating the ring buffer and is expecting a sane state while
+	 * this is true.
+	 */
+	if (rb_resize_disabled(buffer, cpu_id)) {
+		err = -EBUSY;
+		goto out_err_unlock;
+	}
 
+	if (cpu_id == RING_BUFFER_ALL_CPUS) {
 		/* calculate the pages to update */
 		for_each_buffer_cpu(buffer, cpu) {
 			cpu_buffer = buffer->buffers[cpu];
@@ -3409,16 +3421,6 @@ int ring_buffer_resize(struct trace_buffer *buffer, unsigned long size,
 		if (nr_pages == cpu_buffer->nr_pages)
 			goto out;
 
-		/*
-		 * Don't succeed if resizing is disabled, as a reader might be
-		 * manipulating the ring buffer and is expecting a sane state while
-		 * this is true.
-		 */
-		if (atomic_read(&cpu_buffer->resize_disabled)) {
-			err = -EBUSY;
-			goto out_err_unlock;
-		}
-
 		cpu_buffer->nr_pages_to_update = nr_pages -
 						cpu_buffer->nr_pages;
 
@@ -7473,13 +7475,9 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
 
 	old_capacity = rb_subbuf_capacity(buffer);
 
-	/* The mmap fast path reads subbuf_order without buffer->mutex. */
-	for_each_buffer_cpu(buffer, cpu) {
-		if (!cpumask_test_cpu(cpu, buffer->cpumask))
-			continue;
-		if (atomic_read(&buffer->buffers[cpu]->resize_disabled))
-			return -EBUSY;
-	}
+	/* Check it is resizable before we touch subbuf_order */
+	if (rb_resize_disabled(buffer, RING_BUFFER_ALL_CPUS))
+		return -EBUSY;
 
 	atomic_inc(&buffer->record_disabled);
 
@@ -7496,11 +7494,6 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
 
 		cpu_buffer = buffer->buffers[cpu];
 
-		if (atomic_read(&cpu_buffer->resize_disabled)) {
-			err = -EBUSY;
-			goto error;
-		}
-
 		/* Update the number of pages to match the new size */
 		nr_pages = old_capacity * buffer->buffers[cpu]->nr_pages;
 		nr_pages = DIV_ROUND_UP(nr_pages, rb_subbuf_capacity(buffer));
-- 
2.55.0.1082.g2b9226bbc0-goog

[PATCH v2 2/2] ring-buffer: Remove unnecessary cpumask_test_cpu

[Vincent Donnefort]

for_each_buffer_cpu() already iterates over buffer->cpumask. Remove the
unnecessary cpumask_test_cpu() found in ring_buffer_subbuf_order_set().

Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
---
 kernel/trace/ring_buffer.c | 7 -------
 1 file changed, 7 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 73311c4fc556..2087bbfd8f20 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -7488,10 +7488,6 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
 
 	/* Make sure all new buffers are allocated, before deleting the old ones */
 	for_each_buffer_cpu(buffer, cpu) {
-
-		if (!cpumask_test_cpu(cpu, buffer->cpumask))
-			continue;
-
 		cpu_buffer = buffer->buffers[cpu];
 
 		/* Update the number of pages to match the new size */
@@ -7522,9 +7518,6 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
 		struct list_head old_pages;
 		unsigned long flags;
 
-		if (!cpumask_test_cpu(cpu, buffer->cpumask))
-			continue;
-
 		cpu_buffer = buffer->buffers[cpu];
 
 		raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
-- 
2.55.0.1082.g2b9226bbc0-goog