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[PATCH] nbd: fix shutdown and recv work deadlock v2

This fixes a regression added with:

commit e9e006f5fcf2bab59149cb38a48a4817c1b538b4
Author: Mike Christie <mchristi@redhat.com>
Date:   Sun Aug 4 14:10:06 2019 -0500

    nbd: fix max number of supported devs

where we can deadlock during device shutdown. The problem occurs if
the recv_work's nbd_config_put occurs after nbd_start_device_ioctl has
returned and the userspace app has droppped its reference via closing
the device and running nbd_release. The recv_work nbd_config_put call
would then drop the refcount to zero and try to destroy the config which
would try to do destroy_workqueue from the recv work.

This patch just has nbd_start_device_ioctl do a flush_workqueue when it
wakes so we know after the ioctl returns running works have exited. This
also fixes a possible race where we could try to reuse the device while
old recv_works are still running.

Cc: stable@vger.kernel.org
Signed-off-by: Mike Christie <mchristi@redhat.com>
- Drop the taking/dropping of a config_refs around the ioctl. This is
not needed because the caller has incremented the refcount already via
the open() call before doing the ioctl().

 drivers/block/nbd.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 57532465fb83..b4607dd96185 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -1296,10 +1296,10 @@ static int nbd_start_device_ioctl(struct nbd_device *nbd, struct block_device *b
 	ret = wait_event_interruptible(config->recv_wq,
 					 atomic_read(&config->recv_threads) == 0);
-	if (ret) {
+	if (ret)
-		flush_workqueue(nbd->recv_workq);
-	}
+	flush_workqueue(nbd->recv_workq);
 	/* user requested, ignore socket errors */

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