Re: [PATCH 0/4] 2.6.21-rc7 NFS writes: fix a series of issues

Peter Zijlstra
2007-04-29T20:09:43Z

On Sun, 2007-04-29 at 21:41 +0200, Rogier Wolff wrote:
> On Tue, Apr 17, 2007 at 10:37:38PM -0700, Andrew Morton wrote:
> > Florin, can we please see /proc/meminfo as well?
> >
> > Also the result of `echo m > /proc/sysrq-trigger'
>
> Hi,
>
> It's been a while since this thread died out, but maybe I'm
> having the same problem. Networking, large part of memory is
> buffering writes.....
>
> In my case I'm using NBD.
>
> Oh,
>
> /sys/block/nbd0/stat gives:
> 636 88 5353 1700 991 19554 162272 63156 43 1452000 61802352
> I put some debugging stuff in nbd, and it DOES NOT KNOW about the
> 43 requests that the io scheduler claims are in flight at the
> driver....

AFAIK nbd is a tad broken; the following patch used to fix it, although
not in the proper way. Hence it never got merged.

There is a race where the plug state of the device queue gets confused,
which causes requests to just sit on the queue, without further action.

---

Subject: nbd: request_fn fixup

Dropping the queue_lock opens up a nasty race, fix this race by
plugging the device when we're done.

Also includes a small cleanup.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
CC: Daniel Phillips <phillips@google.com>
CC: Pavel Machek <pavel@ucw.cz>
---
drivers/block/nbd.c | 67 ++++++++++++++++++++++++++++++++++++++--------------
1 file changed, 49 insertions(+), 18 deletions(-)

Index: linux-2.6/drivers/block/nbd.c
===================================================================
--- linux-2.6.orig/drivers/block/nbd.c 2006-09-07 17:20:52.000000000 +0200
+++ linux-2.6/drivers/block/nbd.c 2006-09-07 17:35:05.000000000 +0200
@@ -97,20 +97,24 @@ static const char *nbdcmd_to_ascii(int c
}
#endif /* NDEBUG */

-static void nbd_end_request(struct request *req)
+static void __nbd_end_request(struct request *req)
{
int uptodate = (req->errors == 0) ? 1 : 0;
- request_queue_t *q = req->q;
- unsigned long flags;

dprintk(DBG_BLKDEV, "%s: request %p: %s\n", req->rq_disk->disk_name,
req, uptodate? "done": "failed");

- spin_lock_irqsave(q->queue_lock, flags);
- if (!end_that_request_first(req, uptodate, req->nr_sectors)) {
+ if (!end_that_request_first(req, uptodate, req->nr_sectors))
end_that_request_last(req, uptodate);
- }
- spin_unlock_irqrestore(q->queue_lock, flags);
+}
+
+static void nbd_end_request(struct request *req)
+{
+ request_queue_t *q = req->q;
+
+ spin_lock_irq(q->queue_lock);
+ __nbd_end_request(req);
+ spin_unlock_irq(q->queue_lock);
}

/*
@@ -435,10 +439,8 @@ static void do_nbd_request(request_queue
mutex_unlock(&lo->tx_lock);
printk(KERN_ERR "%s: Attempted send on closed socket\n",
lo->disk->disk_name);
- req->errors++;
- nbd_end_request(req);
spin_lock_irq(q->queue_lock);
- continue;
+ goto error_out;
}

lo->active_req = req;
@@ -463,10 +465,13 @@ static void do_nbd_request(request_queue

error_out:
req->errors++;
- spin_unlock(q->queue_lock);
- nbd_end_request(req);
- spin_lock(q->queue_lock);
+ __nbd_end_request(req);
}
+ /*
+ * q->queue_lock has been dropped, this opens up a race
+ * plug the device to close it.
+ */
+ blk_plug_device(q);
return;
}