nbd: fix shutdown and recv work deadlock v2

This fixes a regression added with:

commit 0f7abac3d7496187c0db4e34dbbc29d04cf15786
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
Fixes: 0f7abac3d749 ("nbd: fix max number of supported devs")
Signed-off-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 57532465fb83a7dede4856ddaaf6466f330391d3..b4607dd9618521020bf10ee42c5d3a17b8992d59 100644 (file)
--- 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
        mutex_unlock(&nbd->config_lock);
        ret = wait_event_interruptible(config->recv_wq,
                                         atomic_read(&config->recv_threads) == 0);
-       if (ret) {
+       if (ret)
                sock_shutdown(nbd);
-               flush_workqueue(nbd->recv_workq);
-       }
+       flush_workqueue(nbd->recv_workq);
+
        mutex_lock(&nbd->config_lock);
        nbd_bdev_reset(bdev);
        /* user requested, ignore socket errors */