[Xen-devel] [PATCH] xen/blkback: fix disconnect while I/Os in flight

[Juergen Gross <jgross@xxxxxxxx>]

Today disconnecting xen-blkback is broken in case there are still
I/Os in flight: xen_blkif_disconnect() will bail out early without
releasing all resources in the hope it will be called again when
the last request has terminated. This, however, won't happen as
xen_blkif_free() won't be called on termination of the last running
request: xen_blkif_put() won't decrement the blkif refcnt to 0 as
xen_blkif_disconnect() didn't finish before thus some xen_blkif_put()
calls in xen_blkif_disconnect() didn't happen.

To solve this deadlock xen_blkif_disconnect() and
xen_blkif_alloc_rings() shouldn't use xen_blkif_put() and
xen_blkif_get() but use some other way to do their accounting of
resources.

This at once fixes another error in xen_blkif_disconnect(): when it
returned early with -EBUSY for another ring than 0 it would call
xen_blkif_put() again for already handled rings on a subsequent call.
This will lead to inconsistencies in the refcnt handling.

Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
Tested-by: Steven Haigh <netwiz@xxxxxxxxx>
---
 drivers/block/xen-blkback/common.h | 1 +
 drivers/block/xen-blkback/xenbus.c | 7 +++++--
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/block/xen-blkback/common.h 
b/drivers/block/xen-blkback/common.h
index dea61f6ab8cb..953f38802333 100644
--- a/drivers/block/xen-blkback/common.h
+++ b/drivers/block/xen-blkback/common.h
@@ -281,6 +281,7 @@ struct xen_blkif_ring {
 
        wait_queue_head_t       wq;
        atomic_t                inflight;
+       int                     active;
        /* One thread per blkif ring. */
        struct task_struct      *xenblkd;
        unsigned int            waiting_reqs;
diff --git a/drivers/block/xen-blkback/xenbus.c 
b/drivers/block/xen-blkback/xenbus.c
index 8fe61b5dc5a6..411d2ded2456 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -159,7 +159,7 @@ static int xen_blkif_alloc_rings(struct xen_blkif *blkif)
                init_waitqueue_head(&ring->shutdown_wq);
                ring->blkif = blkif;
                ring->st_print = jiffies;
-               xen_blkif_get(blkif);
+               ring->active = 1;
        }
 
        return 0;
@@ -249,6 +249,9 @@ static int xen_blkif_disconnect(struct xen_blkif *blkif)
                struct xen_blkif_ring *ring = &blkif->rings[r];
                unsigned int i = 0;
 
+               if (!ring->active)
+                       continue;
+
                if (ring->xenblkd) {
                        kthread_stop(ring->xenblkd);
                        wake_up(&ring->shutdown_wq);
@@ -296,7 +299,7 @@ static int xen_blkif_disconnect(struct xen_blkif *blkif)
                BUG_ON(ring->free_pages_num != 0);
                BUG_ON(ring->persistent_gnt_c != 0);
                WARN_ON(i != (XEN_BLKIF_REQS_PER_PAGE * blkif->nr_ring_pages));
-               xen_blkif_put(blkif);
+               ring->active = 0;
        }
        blkif->nr_ring_pages = 0;
        /*
-- 
2.12.0


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