1 files changed, 32 insertions, 2 deletions

diff --git a/kernel/relay.c b/kernel/relay.c
index 04d7cf3ef8cf..d797502140b9 100644
--- a/kernel/relay.c
+++ b/kernel/relay.c
@@ -451,6 +451,13 @@ static struct rchan_buf *relay_open_buf(struct rchan *chan, unsigned int cpu)
                 if (!dentry)
                         goto free_buf;
                 relay_set_buf_dentry(buf, dentry);
+        } else {
+                /* Only retrieve global info, nothing more, nothing less */
+                dentry = chan->cb->create_buf_file(NULL, NULL,
+                                                   S_IRUSR, buf,
+                                                   &chan->is_global);
+                if (WARN_ON(dentry))
+                        goto free_buf;
         }
 
         buf->cpu = cpu;
@@ -562,6 +569,10 @@ static int relay_hotcpu_callback(struct notifier_block *nb,
  *      attributes specified.  The created channel buffer files
  *      will be named base_filename0...base_filenameN-1.  File
  *      permissions will be %S_IRUSR.
+ *
+ *      If opening a buffer (@parent = NULL) that you later wish to register
+ *      in a filesystem, call relay_late_setup_files() once the @parent dentry
+ *      is available.
  */
 struct rchan *relay_open(const char *base_filename,
                          struct dentry *parent,
@@ -640,8 +651,12 @@ static void __relay_set_buf_dentry(void *info)
  *
  *      Returns 0 if successful, non-zero otherwise.
  *
- *      Use to setup files for a previously buffer-only channel.
- *      Useful to do early tracing in kernel, before VFS is up, for example.
+ *      Use to setup files for a previously buffer-only channel created
+ *      by relay_open() with a NULL parent dentry.
+ *
+ *      For example, this is useful for perfomring early tracing in kernel,
+ *      before VFS is up and then exposing the early results once the dentry
+ *      is available.
  */
 int relay_late_setup_files(struct rchan *chan,
                            const char *base_filename,
@@ -666,6 +681,20 @@ int relay_late_setup_files(struct rchan *chan,
         }
         chan->has_base_filename = 1;
         chan->parent = parent;
+
+        if (chan->is_global) {
+                err = -EINVAL;
+                if (!WARN_ON_ONCE(!chan->buf[0])) {
+                        dentry = relay_create_buf_file(chan, chan->buf[0], 0);
+                        if (dentry && !WARN_ON_ONCE(!chan->is_global)) {
+                                relay_set_buf_dentry(chan->buf[0], dentry);
+                                err = 0;
+                        }
+                }
+                mutex_unlock(&relay_channels_mutex);
+                return err;
+        }
+
         curr_cpu = get_cpu();
         /*
          * The CPU hotplug notifier ran before us and created buffers with
@@ -706,6 +735,7 @@ int relay_late_setup_files(struct rchan *chan,
 
         return err;
 }
+EXPORT_SYMBOL_GPL(relay_late_setup_files);
 
 /**
  *      relay_switch_subbuf - switch to a new sub-buffer