[PATCH 08/13] ring-buffer: Add ring_buffer_read_remote_meta_page()

[Vincent Donnefort <vdonnefort@google.com>]

In preparation for the introduction of a panic handler for trace
remotes, add a ring_buffer_read_remote_meta_page(). This is basically
similar to ring_buffer_poll_remote, but it doesn't try to wake-up
readers and, in the !RING_BUFFER_ALL_CPUS case, uses panic-friendly
locks.

While at it, update trace_remote_has_cpu() to use this new function
instead of ring_buffer_poll_remote(), avoiding unnecessary wakeups when
verifying if a CPU buffer is active.

Finally, the distracted engineer who wrote that
ring_buffer_poll_remote() forgot to document it. Add a kerneldoc for
that function too.

Signed-off-by: Vincent Donnefort <vdonnefort@google.com>

diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
index 994f52b34344..6e008a548063 100644
--- a/include/linux/ring_buffer.h
+++ b/include/linux/ring_buffer.h
@@ -298,6 +298,7 @@ struct ring_buffer_remote {
 	void				*priv;
 };
 
+int ring_buffer_read_remote_meta_page(struct trace_buffer *buffer, int cpu);
 int ring_buffer_poll_remote(struct trace_buffer *buffer, int cpu);
 
 struct trace_buffer *
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 7b07d2004cc6..88ac346c65ec 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -6623,6 +6623,59 @@ bool ring_buffer_empty_cpu(struct trace_buffer *buffer, int cpu)
 }
 EXPORT_SYMBOL_GPL(ring_buffer_empty_cpu);
 
+/**
+ * ring_buffer_read_remote_meta_page - read the meta page of a remote ring buffer
+ * @buffer: The ring buffer
+ * @cpu: The CPU buffer to read (or RING_BUFFER_ALL_CPUS)
+ *
+ * Returns:
+ *  0 on success, or -EINVAL if the CPU is not in the buffer's cpumask.
+ */
+int ring_buffer_read_remote_meta_page(struct trace_buffer *buffer, int cpu)
+{
+	struct ring_buffer_per_cpu *cpu_buffer;
+
+	if (cpu != RING_BUFFER_ALL_CPUS) {
+		unsigned long flags;
+		bool dolock;
+
+		if (!cpumask_test_cpu(cpu, buffer->cpumask))
+			return -EINVAL;
+
+		cpu_buffer = buffer->buffers[cpu];
+
+		local_irq_save(flags);
+		dolock = rb_reader_lock(cpu_buffer);
+		rb_read_remote_meta_page(cpu_buffer);
+		rb_reader_unlock(cpu_buffer, dolock);
+		local_irq_restore(flags);
+		return 0;
+	}
+
+	guard(cpus_read_lock)();
+
+	for_each_buffer_cpu(buffer, cpu) {
+		cpu_buffer = buffer->buffers[cpu];
+
+		guard(raw_spinlock)(&cpu_buffer->reader_lock);
+		rb_read_remote_meta_page(cpu_buffer);
+	}
+
+	return 0;
+}
+
+/**
+ * ring_buffer_poll_remote - poll a remote ring buffer for new data
+ * @buffer: The ring buffer
+ * @cpu: The CPU buffer to poll (or RING_BUFFER_ALL_CPUS)
+ *
+ * This function polls the specified remote CPU buffer (or all of them)
+ * by reading its meta page to update the local reader's view. If new
+ * entries are detected, it triggers wakeups for any waiting readers.
+ *
+ * Returns:
+ *  0 on success, or -EINVAL if the CPU is not in the buffer's cpumask.
+ */
 int ring_buffer_poll_remote(struct trace_buffer *buffer, int cpu)
 {
 	struct ring_buffer_per_cpu *cpu_buffer;
diff --git a/kernel/trace/trace_remote.c b/kernel/trace/trace_remote.c
index 1bf0ba159c92..e708dcd7d258 100644
--- a/kernel/trace/trace_remote.c
+++ b/kernel/trace/trace_remote.c
@@ -291,7 +291,7 @@ static bool trace_remote_has_cpu(struct trace_remote *remote, int cpu)
 	if (cpu == RING_BUFFER_ALL_CPUS)
 		return true;
 
-	return ring_buffer_poll_remote(remote->trace_buffer, cpu) == 0;
+	return ring_buffer_read_remote_meta_page(remote->trace_buffer, cpu) == 0;
 }
 
 static void __free_ring_buffer_iter(struct trace_remote_iterator *iter, int cpu)
-- 
2.54.0.1032.g2f8565e1d1-goog