[PATCH] Relay: add CPU hotplug support

Mathieu originally needed to add this for tracing Xen, but it's something
that's needed for any application that can be tracing while cpus are added.

unplug isn't supported by this patch.  The thought was that at minumum a new
buffer needs to be added when a cpu comes up, but it wasn't worth the effort
to remove buffers on cpu down since they'd be freed soon anyway when the
channel was closed.

[zanussi@us.ibm.com: avoid lock_cpu_hotplug deadlock]
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: Tom Zanussi <zanussi@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/filesystems/relay.txt

@@ -157,7 +157,7 @@ TBD(curr. line MT:/API/)
   channel management functions:
 
     relay_open(base_filename, parent, subbuf_size, n_subbufs,
-               callbacks)
+               callbacks, private_data)
     relay_close(chan)
     relay_flush(chan)
     relay_reset(chan)
@@ -251,7 +251,7 @@ static struct rchan_callbacks relay_callbacks =
 
 And an example relay_open() invocation using them:
 
-  chan = relay_open("cpu", NULL, SUBBUF_SIZE, N_SUBBUFS, &relay_callbacks);
+  chan = relay_open("cpu", NULL, SUBBUF_SIZE, N_SUBBUFS, &relay_callbacks, NULL);
 
 If the create_buf_file() callback fails, or isn't defined, channel
 creation and thus relay_open() will fail.
@@ -289,6 +289,11 @@ they use the proper locking for such a buffer, either by wrapping
 writes in a spinlock, or by copying a write function from relay.h and
 creating a local version that internally does the proper locking.
 
+The private_data passed into relay_open() allows clients to associate
+user-defined data with a channel, and is immediately available
+(including in create_buf_file()) via chan->private_data or
+buf->chan->private_data.
+
 Channel 'modes'
 ---------------