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Subject: Re: [v2] nbd: add a flush_workqueue in nbd_start_device
To:     <josef@toxicpanda.com>, <axboe@kernel.dk>, <mchristi@redhat.com>
CC:     <linux-block@vger.kernel.org>, <nbd@other.debian.org>,
        <linux-kernel@vger.kernel.org>
References: <20200122031857.5859-1-sunke32@huawei.com>
From:   "sunke (E)" <sunke32@huawei.com>
Message-ID: <aaa74a5a-3213-7b97-7cc4-89686d985ff2@huawei.com>
Date:   Tue, 4 Feb 2020 10:28:53 +0800
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ping

?? 2020/1/22 11:18, Sun Ke д??:
> When kzalloc fail, may cause trying to destroy the
> workqueue from inside the workqueue.
> 
> If num_connections is m (2 < m), and NO.1 ~ NO.n
> (1 < n < m) kzalloc are successful. The NO.(n + 1)
> failed. Then, nbd_start_device will return ENOMEM
> to nbd_start_device_ioctl, and nbd_start_device_ioctl
> will return immediately without running flush_workqueue.
> However, we still have n recv threads. If nbd_release
> run first, recv threads may have to drop the last
> config_refs and try to destroy the workqueue from
> inside the workqueue.
> 
> To fix it, add a flush_workqueue in nbd_start_device.
> 
> Fixes: e9e006f5fcf2 ("nbd: fix max number of supported devs")
> Signed-off-by: Sun Ke <sunke32@huawei.com>
> ---
>   drivers/block/nbd.c | 10 ++++++++++
>   1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index b4607dd96185..78181908f0df 100644
> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -1265,6 +1265,16 @@ static int nbd_start_device(struct nbd_device *nbd)
>   		args = kzalloc(sizeof(*args), GFP_KERNEL);
>   		if (!args) {
>   			sock_shutdown(nbd);
> +			/*
> +			 * If num_connections is m (2 < m),
> +			 * and NO.1 ~ NO.n(1 < n < m) kzallocs are successful.
> +			 * But NO.(n + 1) failed. We still have n recv threads.
> +			 * So, add flush_workqueue here to prevent recv threads
> +			 * dropping the last config_refs and trying to destroy
> +			 * the workqueue from inside the workqueue.
> +			 */
> +			if (i)
> +				flush_workqueue(nbd->recv_workq);
>   			return -ENOMEM;
>   		}
>   		sk_set_memalloc(config->socks[i]->sock->sk);
>