linux-kernel - Re: [PATCH] quota: Lock s_umount in exclusive mode for Q

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Date:   Fri, 14 Dec 2018 19:49:47 +0100
From:   Javier Barrio <javier.barrio.mart@...il.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Jan Kara <jack@...e.com>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-xfs@...r.kernel.org
Subject: Re: [PATCH] quota: Lock s_umount in exclusive mode for
 Q_XQUOTA{ON,OFF} quotactls.

El 14/12/18 a las 10:56, Jan Kara escribió:

Hi,
>> This patch locks the superblock ->s_umount sem. in exclusive mode for all Q_XQUOTAON/OFF
>> quotactls too in addition to Q_QUOTAON/OFF.
> Thanks for the patch! It looks good to me but let me run it past XFS
> people. Looking at XFS code they definitely do not *need* s_umount in
> exclusive mode for Q_XQUOTAON/OFF (they have their private mutex for
> the exclusion). Shared mode they currently get is enough for them. But
> exclusive mode is fine for them as well AFAICT and it would be easier if
> all quota backends had the same locking rules wrt VFS locks. XFS guys, any
> objections to switching Q_XQUOTAON/OFF handlers from having s_umount locked
> for read to having it locked exclusive?
Thanks, great! I agree.

FWIW, XFS as of now *can* be called while holding s_umount exclusive with the
generic quotactl(Q_QUOTAON/OFF) commands, even though that code path is probably
not exercised in practice (from a quick look xfs-tests/xfs_quota etc always
use the Q_X* variants).

> 								Honza
>
>> AFAICT, other than ext4, only xfs and ocfs2 are affected by this change.
>> The VFS will now call in xfs_quota_* functions with s_umount held, which wasn't the case
>> before. This looks good to me but I can not say for sure. Ext4 and ocfs2 where already
>> beeing called with s_umount exclusive via quota_quotaon/off which is basically the same.
>>
>> Signed-off-by: Javier Barrio <javier.barrio.mart@...il.com>
>> ---
>>
>> [ I'm not familiar with this code, please excuse me if this is not the right fix ]
>>
>>  fs/quota/quota.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/quota/quota.c b/fs/quota/quota.c
>> index f0cbf58ad4da..fd5dd806f1b9 100644
>> --- a/fs/quota/quota.c
>> +++ b/fs/quota/quota.c
>> @@ -791,7 +791,8 @@ static int quotactl_cmd_write(int cmd)
>>  /* Return true if quotactl command is manipulating quota on/off state */
>>  static bool quotactl_cmd_onoff(int cmd)
>>  {
>> -	return (cmd == Q_QUOTAON) || (cmd == Q_QUOTAOFF);
>> +	return (cmd == Q_QUOTAON) || (cmd == Q_QUOTAOFF) ||
>> +		 (cmd == Q_XQUOTAON) || (cmd == Q_XQUOTAOFF);
>>  }
>>  
>>  /*
>> -- 2.17.1
>>
>>