relay: add buffer-only channels; useful for early logging

Allows one to create and use a channel with no associated files.  Files
can be initialized later.  This is useful in scenarios such as logging in
early code, before VFS is up.  Therefore, such channels can be created and
used as soon as kmem_cache_init() completed.

This is needed by kmemtrace to do tracing in early kernel code.

[kosaki.motohiro@jp.fujitsu.com: build fix]
Signed-off-by: Eduard - Gabriel Munteanu <eduard.munteanu@linux360.ro>
Cc: Tom Zanussi <tzanussi@gmail.com>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 141 insertions, 29 deletions

diff --git a/kernel/relay.c b/kernel/relay.c
index 7de644cdec4..04006ef970b 100644
--- a/kernel/relay.c
+++ b/kernel/relay.c
@@ -407,6 +407,35 @@ void relay_reset(struct rchan *chan)
 }
 EXPORT_SYMBOL_GPL(relay_reset);
 
+static inline void relay_set_buf_dentry(struct rchan_buf *buf,
+					struct dentry *dentry)
+{
+	buf->dentry = dentry;
+	buf->dentry->d_inode->i_size = buf->early_bytes;
+}
+
+static struct dentry *relay_create_buf_file(struct rchan *chan,
+					    struct rchan_buf *buf,
+					    unsigned int cpu)
+{
+	struct dentry *dentry;
+	char *tmpname;
+
+	tmpname = kzalloc(NAME_MAX + 1, GFP_KERNEL);
+	if (!tmpname)
+		return NULL;
+	snprintf(tmpname, NAME_MAX, "%s%d", chan->base_filename, cpu);
+
+	/* Create file in fs */
+	dentry = chan->cb->create_buf_file(tmpname, chan->parent,
+					   S_IRUSR, buf,
+					   &chan->is_global);
+
+	kfree(tmpname);
+
+	return dentry;
+}
+
 /*
  *	relay_open_buf - create a new relay channel buffer
  *
@@ -416,45 +445,34 @@ static struct rchan_buf *relay_open_buf(struct rchan *chan, unsigned int cpu)
 {
  	struct rchan_buf *buf = NULL;
 	struct dentry *dentry;
- 	char *tmpname;
 
  	if (chan->is_global)
 		return chan->buf[0];
 
-	tmpname = kzalloc(NAME_MAX + 1, GFP_KERNEL);
- 	if (!tmpname)
- 		goto end;
- 	snprintf(tmpname, NAME_MAX, "%s%d", chan->base_filename, cpu);
-
 	buf = relay_create_buf(chan);
 	if (!buf)
- 		goto free_name;
+		return NULL;
+
+	if (chan->has_base_filename) {
+		dentry = relay_create_buf_file(chan, buf, cpu);
+		if (!dentry)
+			goto free_buf;
+		relay_set_buf_dentry(buf, dentry);
+	}
 
  	buf->cpu = cpu;
  	__relay_reset(buf, 1);
 
-	/* Create file in fs */
- 	dentry = chan->cb->create_buf_file(tmpname, chan->parent, S_IRUSR,
- 					   buf, &chan->is_global);
- 	if (!dentry)
- 		goto free_buf;
-
-	buf->dentry = dentry;
-
  	if(chan->is_global) {
  		chan->buf[0] = buf;
  		buf->cpu = 0;
   	}
 
- 	goto free_name;
+	return buf;
 
 free_buf:
  	relay_destroy_buf(buf);
- 	buf = NULL;
-free_name:
- 	kfree(tmpname);
-end:
-	return buf;
+	return NULL;
 }
 
 /**
@@ -537,8 +555,8 @@ static int __cpuinit relay_hotcpu_callback(struct notifier_block *nb,
 
 /**
  *	relay_open - create a new relay channel
- *	@base_filename: base name of files to create
- *	@parent: dentry of parent directory, %NULL for root directory
+ *	@base_filename: base name of files to create, %NULL for buffering only
+ *	@parent: dentry of parent directory, %NULL for root directory or buffer
  *	@subbuf_size: size of sub-buffers
  *	@n_subbufs: number of sub-buffers
  *	@cb: client callback functions
@@ -560,8 +578,6 @@ struct rchan *relay_open(const char *base_filename,
 {
 	unsigned int i;
 	struct rchan *chan;
-	if (!base_filename)
-		return NULL;
 
 	if (!(subbuf_size && n_subbufs))
 		return NULL;
@@ -576,7 +592,10 @@ struct rchan *relay_open(const char *base_filename,
 	chan->alloc_size = FIX_SIZE(subbuf_size * n_subbufs);
 	chan->parent = parent;
 	chan->private_data = private_data;
-	strlcpy(chan->base_filename, base_filename, NAME_MAX);
+	if (base_filename) {
+		chan->has_base_filename = 1;
+		strlcpy(chan->base_filename, base_filename, NAME_MAX);
+	}
 	setup_callbacks(chan, cb);
 	kref_init(&chan->kref);
 
@@ -604,6 +623,94 @@ free_bufs:
 }
 EXPORT_SYMBOL_GPL(relay_open);
 
+struct rchan_percpu_buf_dispatcher {
+	struct rchan_buf *buf;
+	struct dentry *dentry;
+};
+
+/* Called in atomic context. */
+static void __relay_set_buf_dentry(void *info)
+{
+	struct rchan_percpu_buf_dispatcher *p = info;
+
+	relay_set_buf_dentry(p->buf, p->dentry);
+}
+
+/**
+ *	relay_late_setup_files - triggers file creation
+ *	@chan: channel to operate on
+ *	@base_filename: base name of files to create
+ *	@parent: dentry of parent directory, %NULL for root directory
+ *
+ *	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.
+ */
+int relay_late_setup_files(struct rchan *chan,
+			   const char *base_filename,
+			   struct dentry *parent)
+{
+	int err = 0;
+	unsigned int i, curr_cpu;
+	unsigned long flags;
+	struct dentry *dentry;
+	struct rchan_percpu_buf_dispatcher disp;
+
+	if (!chan || !base_filename)
+		return -EINVAL;
+
+	strlcpy(chan->base_filename, base_filename, NAME_MAX);
+
+	mutex_lock(&relay_channels_mutex);
+	/* Is chan already set up? */
+	if (unlikely(chan->has_base_filename))
+		return -EEXIST;
+	chan->has_base_filename = 1;
+	chan->parent = parent;
+	curr_cpu = get_cpu();
+	/*
+	 * The CPU hotplug notifier ran before us and created buffers with
+	 * no files associated. So it's safe to call relay_setup_buf_file()
+	 * on all currently online CPUs.
+	 */
+	for_each_online_cpu(i) {
+		if (unlikely(!chan->buf[i])) {
+			printk(KERN_ERR "relay_late_setup_files: CPU %u "
+					"has no buffer, it must have!\n", i);
+			BUG();
+			err = -EINVAL;
+			break;
+		}
+
+		dentry = relay_create_buf_file(chan, chan->buf[i], i);
+		if (unlikely(!dentry)) {
+			err = -EINVAL;
+			break;
+		}
+
+		if (curr_cpu == i) {
+			local_irq_save(flags);
+			relay_set_buf_dentry(chan->buf[i], dentry);
+			local_irq_restore(flags);
+		} else {
+			disp.buf = chan->buf[i];
+			disp.dentry = dentry;
+			smp_mb();
+			/* relay_channels_mutex must be held, so wait. */
+			err = smp_call_function_single(i,
+						       __relay_set_buf_dentry,
+						       &disp, 1);
+		}
+		if (unlikely(err))
+			break;
+	}
+	put_cpu();
+	mutex_unlock(&relay_channels_mutex);
+
+	return err;
+}
+
 /**
  *	relay_switch_subbuf - switch to a new sub-buffer
  *	@buf: channel buffer
@@ -627,8 +734,13 @@ size_t relay_switch_subbuf(struct rchan_buf *buf, size_t length)
 		old_subbuf = buf->subbufs_produced % buf->chan->n_subbufs;
 		buf->padding[old_subbuf] = buf->prev_padding;
 		buf->subbufs_produced++;
-		buf->dentry->d_inode->i_size += buf->chan->subbuf_size -
-			buf->padding[old_subbuf];
+		if (buf->dentry)
+			buf->dentry->d_inode->i_size +=
+				buf->chan->subbuf_size -
+				buf->padding[old_subbuf];
+		else
+			buf->early_bytes += buf->chan->subbuf_size -
+					    buf->padding[old_subbuf];
 		smp_mb();
 		if (waitqueue_active(&buf->read_wait))
 			/*
@@ -1237,4 +1349,4 @@ static __init int relay_init(void)
 	return 0;
 }
 
-module_init(relay_init);
+early_initcall(relay_init);