netdev - Re: [RFC PATCH 0/5] nfsd/sunrpc: cleanup resource with sync mode

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Message-ID: <19f1211e-01fe-f2c4-ca3b-c4231e11508c@huaweicloud.com>
Date: Wed, 25 Dec 2024 09:33:20 +0800
From: yangerkun <yangerkun@...weicloud.com>
To: NeilBrown <neilb@...e.de>
Cc: chuck.lever@...cle.com, jlayton@...nel.org, okorniev@...hat.com,
 Dai.Ngo@...cle.com, tom@...pey.com, trondmy@...nel.org, anna@...nel.org,
 linux-nfs@...r.kernel.org, netdev@...r.kernel.org, yangerkun@...wei.com,
 yi.zhang@...wei.com
Subject: Re: [RFC PATCH 0/5] nfsd/sunrpc: cleanup resource with sync mode



在 2024/12/25 6:53, NeilBrown 写道:
> On Tue, 17 Dec 2024, Yang Erkun wrote:
>> From: Yang Erkun <yangerkun@...wei.com>
>>
>> After f8c989a0c89a ("nfsd: release svc_expkey/svc_export with
>> rcu_work"), svc_export_put/expkey_put will call path_put with async
>> mode. This can lead some unexpected failure:
>>
>> mkdir /mnt/sda
>> mkfs.xfs -f /dev/sda
>> echo "/ *(rw,no_root_squash,fsid=0)" > /etc/exports
>> echo "/mnt *(rw,no_root_squash,fsid=1)" >> /etc/exports
>> exportfs -ra
>> service nfs-server start
>> mount -t nfs -o vers=4.0 127.0.0.1:/mnt /mnt1
>> mount /dev/sda /mnt/sda
>> touch /mnt1/sda/file
>> exportfs -r
>> umount /mnt/sda # failed unexcepted
>>
>> The touch above will finally call nfsd_cross_mnt, add refcount to mount,
>> and then add cache_head. Before this commit, exportfs -r will call
>> cache_flush to cleanup all cache_head, and path_put in
>> svc_export_put/expkey_put will be finished with sync mode. So, the
>> latter umount will always success. However, after this commit, path_put
>> will be called with async mode, the latter umount may failed, and if we
>> add some delay, umount will success too. Personally I think this bug and
>> should be fixed. We first revert before bugfix patch, and then fix the
>> original bug with a different way.
> 
> Thanks for these patches.  I think they are certainly a better approach
> to fixing the problem - well done.
> 
> My only thought was that instead of changing how cache_check() works, we
> could introduce cache_check_rcu() which doesn't drop the ref.
> cache_check() would then just call that then optionally drop the ref.
> I'm not convinced that is better, so I'm just mentioning it in case
> anyone else wants to agree.  I'm happy for the patch set to be applied
> as-is.
> 
>   Reviewed-By: NeilBrown <neilb@...e.de>
> 

Thanks a lot for your review!

Yeah, keep cache_check and give a separate function cache_check_rcu will 
make code more clean. Thanks for your advise!

> Thanks,
> NeilBrown