linux-kernel - Re: [PATCH V2] fs/ceph:fix double unlock in handle_cap

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Date:   Wed, 29 Apr 2020 08:46:33 +0800
From:   Wu Bo <wubo40@...wei.com>
To:     Jeff Layton <jlayton@...nel.org>, <sage@...hat.com>,
        <idryomov@...il.com>
CC:     <ceph-devel@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <liuzhiqiang26@...wei.com>, <linfeilong@...wei.com>
Subject: Re: [PATCH V2] fs/ceph:fix double unlock in handle_cap_export()

On 2020/4/28 22:48, Jeff Layton wrote:
> On Tue, 2020-04-28 at 21:13 +0800, Wu Bo wrote:
>> if the ceph_mdsc_open_export_target_session() return fails,
>> should add a lock to avoid twice unlocking.
>> Because the lock will be released at the retry or out_unlock tag.
>>
> 
> The problem looks real, but...
> 
>> --
>> v1 -> v2:
>> add spin_lock(&ci->i_ceph_lock) before goto out_unlock tag.
>>
>> Signed-off-by: Wu Bo <wubo40@...wei.com>
>> ---
>>   fs/ceph/caps.c | 27 +++++++++++++++------------
>>   1 file changed, 15 insertions(+), 12 deletions(-)
>>
>> diff --git a/fs/ceph/caps.c b/fs/ceph/caps.c
>> index 185db76..414c0e2 100644
>> --- a/fs/ceph/caps.c
>> +++ b/fs/ceph/caps.c
>> @@ -3731,22 +3731,25 @@ static void handle_cap_export(struct inode *inode, struct ceph_mds_caps *ex,
>>   
>>   	/* open target session */
>>   	tsession = ceph_mdsc_open_export_target_session(mdsc, target);
>> -	if (!IS_ERR(tsession)) {
>> -		if (mds > target) {
>> -			mutex_lock(&session->s_mutex);
>> -			mutex_lock_nested(&tsession->s_mutex,
>> -					  SINGLE_DEPTH_NESTING);
>> -		} else {
>> -			mutex_lock(&tsession->s_mutex);
>> -			mutex_lock_nested(&session->s_mutex,
>> -					  SINGLE_DEPTH_NESTING);
>> -		}
>> -		new_cap = ceph_get_cap(mdsc, NULL);
>> -	} else {
>> +	if (IS_ERR(tsession)) {
>>   		WARN_ON(1);
>>   		tsession = NULL;
>>   		target = -1;
>> +		mutex_lock(&session->s_mutex);
>> +		spin_lock(&ci->i_ceph_lock);
>> +		goto out_unlock;
> 
> Why did you make this case goto out_unlock instead of retrying as it did
> before?
> 

If the problem occurs, target = -1, and goto retry lable, you need to 
call __get_cap_for_mds() or even call __ceph_remove_cap(), and then jump 
to out_unlock lable. All I think is unnecessary, goto out_unlock instead 
of retrying directly.

Thanks.
Wu Bo

>> +	}
>> +
>> +	if (mds > target) {
>> +		mutex_lock(&session->s_mutex);
>> +		mutex_lock_nested(&tsession->s_mutex,
>> +					SINGLE_DEPTH_NESTING);
>> +	} else {
>> +		mutex_lock(&tsession->s_mutex);
>> +		mutex_lock_nested(&session->s_mutex,
>> +					SINGLE_DEPTH_NESTING);
>>   	}
>> +	new_cap = ceph_get_cap(mdsc, NULL);
>>   	goto retry;
>>   
>>   out_unlock:
>