I guess they might have to switch to such an asynchronous delivery
system if they want to do this properly. Simply put, your polling
solution is exactly what I do, but I check a flag set by the writer
instead of waking up the readers unconditionally.

Mathieu
  

Ingo's solution could call waitqueue_active() inside wakeup_readers() to 
determine if there are waiters. Right?

quoted

	Ingo

------------------------------------->
Subject: relay: fix timer madness
From: Ingo Molnar <redacted>

remove timer calls (!!!) from deep within the tracing infrastructure.
This was totally bogus code that can cause lockups and worse.
Poll the buffer every 2 jiffies for now.

Signed-off-by: Ingo Molnar <redacted>
---
 kernel/relay.c |   14 +++++---------
 1 file changed, 5 insertions(+), 9 deletions(-)

Index: linux-rt-rebase.q/kernel/relay.c
===================================================================

--- linux-rt-rebase.q.orig/kernel/relay.c
+++ linux-rt-rebase.q/kernel/relay.c

@@ -319,6 +319,10 @@ static void wakeup_readers(unsigned long
 {
 	struct rchan_buf *buf = (struct rchan_buf *)data;
 	wake_up_interruptible(&buf->read_wait);
+	/*
+	 * Stupid polling for now:
+	 */
+	mod_timer(&buf->timer, jiffies + 1);
 }
 
 /**

@@ -336,6 +340,7 @@ static void __relay_reset(struct rchan_b
 		init_waitqueue_head(&buf->read_wait);
 		kref_init(&buf->kref);
 		setup_timer(&buf->timer, wakeup_readers, (unsigned long)buf);
+		mod_timer(&buf->timer, jiffies + 1);
 	} else
 		del_timer_sync(&buf->timer);
 

@@ -604,15 +609,6 @@ size_t relay_switch_subbuf(struct rchan_
 		buf->subbufs_produced++;
 		buf->dentry->d_inode->i_size += buf->chan->subbuf_size -
 			buf->padding[old_subbuf];
-		smp_mb();
-		if (waitqueue_active(&buf->read_wait))
-			/*
-			 * Calling wake_up_interruptible() from here
-			 * will deadlock if we happen to be logging
-			 * from the scheduler (trying to re-grab
-			 * rq->lock), so defer it.
-			 */
-			__mod_timer(&buf->timer, jiffies + 1);
 	}
 
 	old = buf->data;