[PATCH] relayfs: remove warning printk() in relay_switch_subbuf()

There's currently a diagnostic printk in relay_switch_subbuf() meant as
a warning if you accidentally try to log an event larger than the
sub-buffer size.

The problem is if this happens while logging from somewhere it's not
safe to be doing printks, such as in the scheduler, you can end up with
a deadlock.  This patch removes the warning from relay_switch_subbuf()
and instead prints some diagnostic info when the channel is closed.

Thanks to Mathieu Desnoyers for pointing out the problem and
suggesting a fix.

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

fs/relayfs/relay.c

@@ -333,8 +333,7 @@ size_t relay_switch_subbuf(struct rchan_buf *buf, size_t length)
 	return length;
 
 toobig:
-	printk(KERN_WARNING "relayfs: event too large (%Zd)\n", length);
+	buf->chan->last_toobig = length;
-	WARN_ON(1);
 	return 0;
 }
 
@@ -399,6 +398,11 @@ void relay_close(struct rchan *chan)
 		relay_close_buf(chan->buf[i]);
 	}
 
+	if (chan->last_toobig)
+		printk(KERN_WARNING "relayfs: one or more items not logged "
+		       "[item size (%Zd) > sub-buffer size (%Zd)]\n",
+		       chan->last_toobig, chan->subbuf_size);
+
 	kref_put(&chan->kref, relay_destroy_channel);
 }
 

include/linux/relayfs_fs.h

@@ -20,9 +20,9 @@
 #include <linux/kref.h>
 
 /*
- * Tracks changes to rchan_buf struct
+ * Tracks changes to rchan/rchan_buf structs
  */
-#define RELAYFS_CHANNEL_VERSION		5
+#define RELAYFS_CHANNEL_VERSION		6
 
 /*
  * Per-cpu relay channel buffer
@@ -60,6 +60,7 @@ struct rchan
 	struct rchan_callbacks *cb;	/* client callbacks */
 	struct kref kref;		/* channel refcount */
 	void *private_data;		/* for user-defined data */
+	size_t last_toobig;		/* tried to log event > subbuf size */
 	struct rchan_buf *buf[NR_CPUS]; /* per-cpu channel buffers */
 };