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Subject: Re: [PATCH] nbd: add a flush_workqueue in nbd_start_device
To:     Jens Axboe <axboe@kernel.dk>, Josef Bacik <josef@toxicpanda.com>,
        <mchristi@redhat.com>
CC:     <linux-block@vger.kernel.org>, <nbd@other.debian.org>,
        <linux-kernel@vger.kernel.org>
References: <20200121124813.13332-1-sunke32@huawei.com>
 <82a3eb7e-883c-a091-feec-27f3937491ab@toxicpanda.com>
 <83d21549-66a0-0e76-89e5-1303c5b19102@kernel.dk>
From:   "sunke (E)" <sunke32@huawei.com>
Message-ID: <a180649a-d73b-24ad-14d5-d3ed992bba0d@huawei.com>
Date:   Wed, 22 Jan 2020 10:45:39 +0800
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在 2020/1/22 5:25, Jens Axboe 写道:
> On 1/21/20 7:00 AM, Josef Bacik wrote:
>> On 1/21/20 7:48 AM, Sun Ke wrote:
>>> 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 | 7 ++++++-
>>>    1 file changed, 6 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
>>> index b4607dd96185..dd1f8c2c6169 100644
>>> --- a/drivers/block/nbd.c
>>> +++ b/drivers/block/nbd.c
>>> @@ -1264,7 +1264,12 @@ static int nbd_start_device(struct nbd_device *nbd)
>>>    
>>>    		args = kzalloc(sizeof(*args), GFP_KERNEL);
>>>    		if (!args) {
>>> -			sock_shutdown(nbd);
>>> +			if (i == 0)
>>> +				sock_shutdown(nbd);
>>> +			else {
>>> +				sock_shutdown(nbd);
>>> +				flush_workqueue(nbd->recv_workq);
>>> +			}
>>
>> Just for readability sake why don't we just flush_workqueue()
>> unconditionally, and add a comment so we know why in the future.
> 
> Or maybe just make it:
> 
> 	sock_shutdown(nbd);
> 	if (i)
> 		flush_workqueue(nbd->recv_workq);
> 
> which does the same thing, but is still readable. The current code with
> the shutdown duplication is just a bit odd. Needs a comment either way.
> 

OK, I will improve it in my v2 patch.

Thanks,

Sun Ke