On Mon, May 05 2008, Neil Brown wrote:

On Sunday May 4, jens.axboe@oracle.com wrote:

quoted

On Sun, May 04 2008, Jacek Luczak wrote:

quoted

Hi,

I've CC:-ed few guys which may help.

Prakash Punnoor pisze:

quoted

Hi, I got this on boot:

usb 2-1.3: new full speed USB device using ehci_hcd and address 3
usb 2-1.3: configuration #1 chosen from 1 choice
Clocksource tsc unstable (delta = -117343945 ns)
------------[ cut here ]------------
WARNING: at include/linux/blkdev.h:443 blk_remove_plug+0x7d/0x90()

...

quoted

Looks like it caught a real bug there - unfortunately we have to check
for ->queue_lock here as well, if this is another stacked devices and
not the bottom device. Does this make the warning go away for you?

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 087eee0..958f26b 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c

@@ -3264,6 +3264,8 @@ static void raid5_unplug_device(struct request_queue *q)
 	unsigned long flags;
 
 	spin_lock_irqsave(&conf->device_lock, flags);
+	if (q->queue_lock)
+		spin_lock(q->queue_lock);
 
 	if (blk_remove_plug(q)) {
 		conf->seq_flush++;

@@ -3271,6 +3273,8 @@ static void raid5_unplug_device(struct request_queue *q)
 	}
 	md_wakeup_thread(mddev->thread);
 
+	if (q->queue_lock)
+		spin_unlock(q->queue_lock);
 	spin_unlock_irqrestore(&conf->device_lock, flags);
 
 	unplug_slaves(mddev);

I suspect that will just cause more problems, as the 'q' for an md
device never gets ->queue_lock initialised.
I suspect the correct thing to do is set
	q->queue_lock = &conf->device_lock;

at some stage, probably immediately after device_lock is initialised
in 'run'.

I was discussing this with Dan Williams starting
  http://marc.info/?l=linux-raid&m=120951839903995&w=4
though we don't have an agreed patch yet.

I agree with the usage of the device lock. I (mistakenly) thought that
raid5 used the bottom device queue for that unplug - I see that it does
not, so where does the warning come from? mddev->queue->queue_lock
should be NULL, since md never sets it and it's zeroed to begin with??

I'm wondering why you mention the issues of stacked devices though.  I
don't see how it applies.  Could you explain?

See above, if the queue had been the bottom queue, ->queue_lock may or
may not be NULL depending on whether this is the real device or
(another) stacked device.

-- 
Jens Axboe