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SubjectRe: [PATCH] 2.6.0 NBD driver: remove send/recieve race for request
Paul Clements wrote:

>. . .
>Here's the patch to fix up several race conditions in nbd. It requires
>reverting the already included (but admittedly incomplete)
>nbd-race-fix.patch that's in -mm5.
>
>Andrew, please apply.
>
>Thanks,
>Paul
>
>------------------------------------------------------------------------
>
>--- linux-2.6.0-test2-mm4-PRISTINE/drivers/block/nbd.c Sun Jul 27 12:58:51 2003
>+++ linux-2.6.0-test2-mm4/drivers/block/nbd.c Thu Aug 7 18:02:23 2003
>@@ -416,11 +416,19 @@ void nbd_clear_que(struct nbd_device *lo
> BUG_ON(lo->magic != LO_MAGIC);
> #endif
>
>+retry:
> do {
> req = NULL;
> spin_lock(&lo->queue_lock);
> if (!list_empty(&lo->queue_head)) {
> req = list_entry(lo->queue_head.next, struct request, queuelist);
>+ if (req->ref_count > 1) { /* still in xmit */
>+ spin_unlock(&lo->queue_lock);
>+ printk(KERN_DEBUG "%s: request %p: still in use (%d), waiting...\n",
>+ lo->disk->disk_name, req, req->ref_count);
>+ schedule_timeout(HZ); /* wait a second */
>
Isn't there something more deterministic than just waiting a second and
hoping things clear up that you can use here? How about not clearing the
queue unless lo->sock is NULL and using whatever lock it is now that's
protecting lo->sock. That way the queue clearing race can be eliminated too.

>+ goto retry;
>+ }
> list_del_init(&req->queuelist);
> }
> spin_unlock(&lo->queue_lock);
>@@ -490,6 +498,7 @@ static void do_nbd_request(request_queue
> }
>
> list_add(&req->queuelist, &lo->queue_head);
>+ req->ref_count++; /* make sure req does not get freed */
> spin_unlock(&lo->queue_lock);
>
> nbd_send_req(lo, req);
>@@ -499,12 +508,14 @@ static void do_nbd_request(request_queue
> lo->disk->disk_name);
> spin_lock(&lo->queue_lock);
> list_del_init(&req->queuelist);
>+ req->ref_count--;
> spin_unlock(&lo->queue_lock);
> nbd_end_request(req);
> spin_lock_irq(q->queue_lock);
> continue;
> }
>
>+ req->ref_count--;
> spin_lock_irq(q->queue_lock);
>
Since ref_count isn't atomic, shouldn't ref_count only be changed while
the queue_lock is held???


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