[PATCH] relayfs: add support for global relay buffers

This patch adds the optional is_global outparam to the create_buf_file()
callback.  This can be used by clients to create a single global relayfs
buffer instead of the default per-cpu buffers.  This was suggested as being
useful for certain debugging applications where it's more convenient to be
able to get all the data from a single channel without having to go to the
bother of dealing with per-cpu files.

Signed-off-by: Tom Zanussi <zanussi@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

2 files changed, 32 insertions, 11 deletions

diff --git a/fs/relayfs/relay.c b/fs/relayfs/relay.c
index b9bb56903272..2935a6ab8ffa 100644
--- a/fs/relayfs/relay.c
+++ b/fs/relayfs/relay.c
@@ -86,7 +86,8 @@ static void buf_unmapped_default_callback(struct rchan_buf *buf,
 static struct dentry *create_buf_file_default_callback(const char *filename,
                                                        struct dentry *parent,
                                                        int mode,
-                                                       struct rchan_buf *buf)
+                                                       struct rchan_buf *buf,
+                                                       int *is_global)
 {
         return relayfs_create_file(filename, parent, mode,
                                    &relayfs_file_operations, buf);
@@ -170,14 +171,16 @@ static inline void __relay_reset(struct rchan_buf *buf, unsigned int init)
 void relay_reset(struct rchan *chan)
 {
         unsigned int i;
+        struct rchan_buf *prev = NULL;
 
         if (!chan)
                 return;
 
         for (i = 0; i < NR_CPUS; i++) {
-                if (!chan->buf[i])
-                        continue;
+                if (!chan->buf[i] || chan->buf[i] == prev)
+                        break;
                 __relay_reset(chan->buf[i], 0);
+                prev = chan->buf[i];
         }
 }
 
@@ -188,18 +191,22 @@ void relay_reset(struct rchan *chan)
  */
 static struct rchan_buf *relay_open_buf(struct rchan *chan,
                                         const char *filename,
-                                        struct dentry *parent)
+                                        struct dentry *parent,
+                                        int *is_global)
 {
         struct rchan_buf *buf;
         struct dentry *dentry;
 
+        if (*is_global)
+                return chan->buf[0];
+
         buf = relay_create_buf(chan);
         if (!buf)
                 return NULL;
 
         /* Create file in fs */
         dentry = chan->cb->create_buf_file(filename, parent, S_IRUSR,
-                                           buf);
+                                           buf, is_global);
         if (!dentry) {
                 relay_destroy_buf(buf);
                 return NULL;
@@ -273,6 +280,7 @@ struct rchan *relay_open(const char *base_filename,
         unsigned int i;
         struct rchan *chan;
         char *tmpname;
+        int is_global = 0;
 
         if (!base_filename)
                 return NULL;
@@ -297,7 +305,8 @@ struct rchan *relay_open(const char *base_filename,
 
         for_each_online_cpu(i) {
                 sprintf(tmpname, "%s%d", base_filename, i);
-                chan->buf[i] = relay_open_buf(chan, tmpname, parent);
+                chan->buf[i] = relay_open_buf(chan, tmpname, parent,
+                                              &is_global);
                 chan->buf[i]->cpu = i;
                 if (!chan->buf[i])
                         goto free_bufs;
@@ -311,6 +320,8 @@ free_bufs:
                 if (!chan->buf[i])
                         break;
                 relay_close_buf(chan->buf[i]);
+                if (is_global)
+                        break;
         }
         kfree(tmpname);
 
@@ -420,14 +431,16 @@ void relay_destroy_channel(struct kref *kref)
 void relay_close(struct rchan *chan)
 {
         unsigned int i;
+        struct rchan_buf *prev = NULL;
 
         if (!chan)
                 return;
 
         for (i = 0; i < NR_CPUS; i++) {
-                if (!chan->buf[i])
-                        continue;
+                if (!chan->buf[i] || chan->buf[i] == prev)
+                        break;
                 relay_close_buf(chan->buf[i]);
+                prev = chan->buf[i];
         }
 
         if (chan->last_toobig)
@@ -447,14 +460,16 @@ void relay_close(struct rchan *chan)
 void relay_flush(struct rchan *chan)
 {
         unsigned int i;
+        struct rchan_buf *prev = NULL;
 
         if (!chan)
                 return;
 
         for (i = 0; i < NR_CPUS; i++) {
-                if (!chan->buf[i])
-                        continue;
+                if (!chan->buf[i] || chan->buf[i] == prev)
+                        break;
                 relay_switch_subbuf(chan->buf[i], 0);
+                prev = chan->buf[i];
         }
 }
 
diff --git a/include/linux/relayfs_fs.h b/include/linux/relayfs_fs.h
index 8c2177105857..30f45511b40d 100644
--- a/include/linux/relayfs_fs.h
+++ b/include/linux/relayfs_fs.h
@@ -116,6 +116,7 @@ struct rchan_callbacks
          * @parent: the parent of the file to create
          * @mode: the mode of the file to create
          * @buf: the channel buffer
+         * @is_global: outparam - set non-zero if the buffer should be global
          *
          * Called during relay_open(), once for each per-cpu buffer,
          * to allow the client to create a file to be used to
@@ -126,12 +127,17 @@ struct rchan_callbacks
          * The callback should return the dentry of the file created
          * to represent the relay buffer.
          *
+         * Setting the is_global outparam to a non-zero value will
+         * cause relay_open() to create a single global buffer rather
+         * than the default set of per-cpu buffers.
+         *
          * See Documentation/filesystems/relayfs.txt for more info.
          */
         struct dentry *(*create_buf_file)(const char *filename,
                                           struct dentry *parent,
                                           int mode,
-                                          struct rchan_buf *buf);
+                                          struct rchan_buf *buf,
+                                          int *is_global);
 
         /*
          * remove_buf_file - remove file representing a relayfs channel buffer