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SubjectRe: [PATCH 2.4.22-pre] nbd: fix race conditions
In article <iKef.8c1.15@gated-at.bofh.it> you wrote:
> This patch is similar to one I posted yesterday for 2.6. It fixes the
> following race conditions in nbd:
>
> 1) adds locking and properly orders the code in NBD_CLEAR_SOCK to
> eliminate races with other code

This is not so clear as for the 2.6 code. There's no ref_count in the
2.4 request struct, so you have no standard way to mark a request as
in-flight, and hence no standard way to get clr_queue to skip it for the
moment. You are using something else as a marker, but I'm not quite
clear what. Can you say?

To summarise - the patch below appears to me not to do what you say it
does, though it does cure races between setting and unsetting lo->file
and lo->sock. It doesn't appear to cure races between clearing requests and
doing them.

> 2) adds an lo->sock check to nbd_clear_que to eliminate races between
> do_nbd_request and nbd_clear_que, which resulted in the dequeuing of
> active requests
>
> 3) adds an lo->sock check to NBD_DO_IT to eliminate races with
> NBD_CLEAR_SOCK, which caused an Oops when "nbd-client -d" was called
>

All these are for the same thing - the metadataraces.

> --- linux-2.4.21-PRISTINE/drivers/block/nbd.c Fri Jun 13 10:51:32 2003
> +++ linux-2.4.21/drivers/block/nbd.c Sat Aug 9 13:25:49 2003
> @@ -428,23 +428,24 @@ static int nbd_ioctl(struct inode *inode
> return 0 ;
>
> case NBD_CLEAR_SOCK:
> + error = 0;
> + down(&lo->tx_lock);
> + lo->sock = NULL;
> + up(&lo->tx_lock);

Unggg .. you mark lo->sock as null, atomically.

> + spin_lock(&lo->queue_lock);
> + file = lo->file;
> + lo->file = NULL;
> + spin_unlock(&lo->queue_lock);

You save lo->file and mark it null, atomically.

> nbd_clear_que(lo);
> spin_lock(&lo->queue_lock);
> if (!list_empty(&lo->queue_head)) {
> - spin_unlock(&lo->queue_lock);
> - printk(KERN_ERR "nbd: Some requests are in progress -> can not turn off.\n");
> - return -EBUSY;
> - }

You remove the code that gave up on trying to clear socket if the queue is
nonempty. Why? What's wrong with getting userspace to retry until OK?
Oh, I see ...

> - file = lo->file;
> - if (!file) {
> - spin_unlock(&lo->queue_lock);
> - return -EINVAL;
> + printk(KERN_ERR "nbd: disconnect: some requests are in progress -> please try again.\n");
> + error = -EBUSY;
> }

... it looks as though if the queue is nonempty, we now still return
busy. So what you did was remove the test on lo->file. Now we don't
care about the lo->file state. Why?


> - lo->file = NULL;
> - lo->sock = NULL;
> spin_unlock(&lo->queue_lock);
> - fput(file);
> - return 0;
> + if (file)
> + fput(file);
> + return error;

Well, that was harmless.

> case NBD_SET_SOCK:
> if (lo->file)
> return -EBUSY;
> @@ -491,9 +492,12 @@ static int nbd_ioctl(struct inode *inode
> * there should be a more generic interface rather than
> * calling socket ops directly here */
> down(&lo->tx_lock);
> - printk(KERN_WARNING "nbd: shutting down socket\n");
> - lo->sock->ops->shutdown(lo->sock, SEND_SHUTDOWN|RCV_SHUTDOWN);
> - lo->sock = NULL;
> + if (lo->sock) {
> + printk(KERN_WARNING "nbd: shutting down socket\n");
> + lo->sock->ops->shutdown(lo->sock,
> + SEND_SHUTDOWN|RCV_SHUTDOWN);
> + lo->sock = NULL;
> + }

Harmless.

> up(&lo->tx_lock);
> spin_lock(&lo->queue_lock);
> file = lo->file;
> @@ -505,6 +509,13 @@ static int nbd_ioctl(struct inode *inode
> fput(file);
> return lo->harderror;
> case NBD_CLEAR_QUE:
> + down(&lo->tx_lock);
> + if (lo->sock) {
> + up(&lo->tx_lock);
> + return 0; /* probably should be error, but that would
> + * break "nbd-client -d", so just return 0 */
> + }
> + up(&lo->tx_lock);

You don't clear queue while lo->sock is nonzero. This means that one
must have cleared socket beforehand.

> nbd_clear_que(lo);
> return 0;
> #ifdef PARANOIA

I don't see any race conditions going away. Except possibly a race
between set sock and clear sock to set lo->sock. The setting of
lo->file is also a little more atomic now, and that's OK. But surely
the race condition you cured in 2.6 was not this at all, but a race
between clearing the queue and doing the requests?

Peter
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