ring-buffer updates for v7.4:

- Add Vincent Donnefort as ring-buffer maintainer
 
   Vincent has been doing a lot of updates to the ftrace ring buffer lately.
   I'm going to start delegating updates for the ring buffer to him.
 
 - Use RING_BUFFER_MIN_NR_PAGES instead of open coding the number 2
 
   Instead of using the number 2 as the minimum number of pages that a ring
   buffer may have, create and use a macro RING_BUFFER_MIN_NR_PAGES.
 
 - Clean up the resize_disabled_checks
 
   There's duplication of code to check for resize disabled. Create a helper
   function rb_resize_disabled() and use that. Not only does it put the code
   in one place, it also removed a redundant check.
 
 - Remove unnecessary cpumask_test_cpu checks
 
   The loop for_each_buffer_cpu(buffer, cpu) only loops over CPUs in the
   buffer->cpumask. There's no reason to test if the CPU is set in that same
   loop.
 
 - Take cpus_read_lock() when changing the subbuf order
 
   A CPU coming on line for the first time allocates its CPU buffer. But
   if this happens while the subbuffer is being resized, it can cause an
   inconsistent state of subbufer sizes in the buffer. Add a cpus_read_lock()
   to ring_buffer_subbuf_order_set() to prevent this race.
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Merge tag 'trace-ringbuffer-v7.4' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace

Pull ring-buffer updates from Steven Rostedt:

 - Add Vincent Donnefort as ring-buffer maintainer

   Vincent has been doing a lot of updates to the ftrace ring buffer
   lately. I'm going to start delegating updates for the ring buffer to
   him.

 - Use RING_BUFFER_MIN_NR_PAGES instead of open coding the number 2

   Instead of using the number 2 as the minimum number of pages that a
   ring buffer may have, create and use a macro
   RING_BUFFER_MIN_NR_PAGES.

 - Clean up the resize_disabled_checks

   There's duplication of code to check for resize disabled. Create a
   helper function rb_resize_disabled() and use that. Not only does it
   put the code in one place, it also removed a redundant check.

 - Remove unnecessary cpumask_test_cpu checks

   The loop for_each_buffer_cpu(buffer, cpu) only loops over CPUs in the
   buffer->cpumask. There's no reason to test if the CPU is set in that
   same loop.

 - Take cpus_read_lock() when changing the subbuf order

   A CPU coming on line for the first time allocates its CPU buffer. But
   if this happens while the subbuffer is being resized, it can cause an
   inconsistent state of subbufer sizes in the buffer. Add a
   cpus_read_lock() to ring_buffer_subbuf_order_set() to prevent this
   race.

* tag 'trace-ringbuffer-v7.4' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace:
  ring-buffer: Take cpus_read_lock() when changing the subbuf order
  ring-buffer: Remove unnecessary cpumask_test_cpu
  ring-buffer: Clean up resize_disabled checks
  ring-buffer: Unify ring buffer minimum page calculations
  ring-buffer: Add a MAINTAINERS category specific for the tracing ring-buffer

MAINTAINERS

@@ -27984,6 +27984,22 @@ F:	kernel/trace/trace_irqsoff.c
 F:	kernel/trace/trace_osnoise.c
 F:	kernel/trace/trace_sched_wakeup.c
 
+TRACING RING BUFFER
+M:	Steven Rostedt <rostedt@goodmis.org>
+M:	Vincent Donnefort <vdonnefort@google.com>
+M:	Masami Hiramatsu <mhiramat@kernel.org>
+L:	linux-kernel@vger.kernel.org
+L:	linux-trace-kernel@vger.kernel.org
+S:	Maintained
+Q:	https://patchwork.kernel.org/project/linux-trace-kernel/list/
+T:	git git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace.git
+F:	Documentation/trace/ring-buffer*
+F:	Documentation/trace/remotes.rst
+F:	include/linux/ring-buffer*.h
+F:	kernel/trace/*ring_buffer*.c
+F:	kernel/trace/remote*.c
+F:	tools/testing/selftests/ring-buffer/
+
 TRADITIONAL CHINESE DOCUMENTATION
 M:	Chen-Yu Yeh <chenyou910331@gmail.com>
 M:	Weijie Yuan <wy@wyuan.org>

include/linux/ring_buffer.h

@@ -98,6 +98,13 @@ struct trace_buffer *__ring_buffer_alloc_range(unsigned long size, unsigned flag
 
 void *ring_buffer_meta_scratch(struct trace_buffer *buffer, unsigned int *size);
 
+/*
+ * A minimum of 2 pages ensures we can always swap a page out of the ring
+ * buffer. This does not account for the reader page or any other page
+ * containing meta data used by static ring buffers.
+ */
+#define RING_BUFFER_MIN_NR_PAGES 2UL
+
 /*
  * Because the ring buffer is generic, if other users of the ring buffer get
  * traced by ftrace, it can produce lockdep warnings. We need to keep each
@@ -287,7 +294,7 @@ static inline struct ring_buffer_desc *__first_ring_buffer_desc(struct trace_buf
 static inline unsigned long __calc_nr_pages_ring_buffer_desc(size_t size)
 {
 	/* Takes into account the reader page */
-	return max(DIV_ROUND_UP(size, PAGE_SIZE - BUF_PAGE_HDR_SIZE), 2UL) + 1;
+	return max(DIV_ROUND_UP(size, PAGE_SIZE - BUF_PAGE_HDR_SIZE), RING_BUFFER_MIN_NR_PAGES) + 1;
 }
 
 static inline size_t trace_buffer_desc_size(size_t buffer_size, unsigned int nr_cpus)

kernel/trace/ring_buffer.c

@@ -642,6 +642,11 @@ static __always_inline unsigned int rb_subbuf_max_data_size(struct trace_buffer
 	return rb_subbuf_capacity(buffer) - RB_EVNT_HDR_SIZE - sizeof(event->array[0]);
 }
 
+static unsigned long rb_calculate_nr_pages(struct trace_buffer *buffer, unsigned long size)
+{
+	return max(DIV_ROUND_UP(size, rb_subbuf_capacity(buffer)), RING_BUFFER_MIN_NR_PAGES);
+}
+
 /**
  * rb_subbuf_start - Get the start address of a subbuffer
  * @buffer:	A trace buffer
@@ -2868,7 +2873,7 @@ static struct trace_buffer *alloc_buffer(unsigned long size, unsigned flags,
 			goto fail_free_buffers;
 
 		/* Need at least two pages plus the reader page */
-		if (nr_pages < 3)
+		if (nr_pages < RING_BUFFER_MIN_NR_PAGES + 1)
 			goto fail_free_buffers;
 
  again:
@@ -2880,7 +2885,7 @@ static struct trace_buffer *alloc_buffer(unsigned long size, unsigned flags,
 			ptr += subbuf_size * nr_pages;
 		}
 		if (ptr > end) {
-			if (nr_pages <= 3)
+			if (nr_pages <= RING_BUFFER_MIN_NR_PAGES + 1)
 				goto fail_free_buffers;
 			nr_pages--;
 			goto again;
@@ -2903,13 +2908,10 @@ static struct trace_buffer *alloc_buffer(unsigned long size, unsigned flags,
 		if (nr_pages > rb_static_max_pages())
 			goto fail_free_buffers;
 
-		if (nr_pages < 2)
+		if (nr_pages < RING_BUFFER_MIN_NR_PAGES)
 			goto fail_free_buffers;
 	} else {
-		/* need at least two pages */
-		nr_pages = DIV_ROUND_UP(size, rb_subbuf_capacity(buffer));
-		if (nr_pages < 2)
-			nr_pages = 2;
+		nr_pages = rb_calculate_nr_pages(buffer, size);
 	}
 
 	cpumask_set_cpu(cpu, buffer->cpumask);
@@ -3284,6 +3286,21 @@ 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.
@@ -3322,26 +3339,19 @@ int ring_buffer_resize(struct trace_buffer *buffer, unsigned long size,
 	mutex_lock(&buffer->mutex);
 	atomic_inc(&buffer->resizing);
 
-	nr_pages = DIV_ROUND_UP(size, rb_subbuf_capacity(buffer));
+	nr_pages = rb_calculate_nr_pages(buffer, size);
 
-	/* we need a minimum of two pages */
-	if (nr_pages < 2)
-		nr_pages = 2;
+	/*
+	 * 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) {
-		/*
-		 * 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;
-			}
-		}
-
 		/* calculate the pages to update */
 		for_each_buffer_cpu(buffer, cpu) {
 			cpu_buffer = buffer->buffers[cpu];
@@ -3413,16 +3423,6 @@ 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;
 
@@ -7466,6 +7466,8 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
 	if (psize > RB_WRITE_MASK + 1)
 		return -EINVAL;
 
+	guard(cpus_read_lock)();
+
 	/* prevent another thread from changing buffer sizes */
 	guard(mutex)(&buffer->mutex);
 
@@ -7475,13 +7477,9 @@ 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);
 
@@ -7492,24 +7490,11 @@ 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];
 
-		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));
-
-		/* we need a minimum of two pages */
-		if (nr_pages < 2)
-			nr_pages = 2;
+		nr_pages = rb_calculate_nr_pages(buffer,
+						 old_capacity * buffer->buffers[cpu]->nr_pages);
 
 		cpu_buffer->nr_pages_to_update = nr_pages;
 
@@ -7531,9 +7516,6 @@ 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);
@@ -8100,7 +8082,7 @@ int trace_rb_cpu_prepare(unsigned int cpu, struct hlist_node *node)
 	}
 	/* allocate minimum pages, user can later expand it */
 	if (!nr_pages_same)
-		nr_pages = 2;
+		nr_pages = RING_BUFFER_MIN_NR_PAGES;
 	buffer->buffers[cpu] =
 		rb_allocate_cpu_buffer(buffer, nr_pages, cpu);
 	if (!buffer->buffers[cpu]) {

kernel/trace/simple_ring_buffer.c

@@ -383,8 +383,8 @@ int simple_ring_buffer_init_mm(struct simple_rb_per_cpu *cpu_buffer,
 	void *page;
 	int i;
 
-	/* At least 1 reader page and two pages in the ring-buffer */
-	if (desc->nr_page_va < 3)
+	/* At least RING_BUFFER_MIN_NR_PAGES and 1 reader page */
+	if (desc->nr_page_va < RING_BUFFER_MIN_NR_PAGES + 1)
 		return -EINVAL;
 
 	memset(cpu_buffer, 0, sizeof(*cpu_buffer));