Re: [Ocfs2-devel] [PATCH v2 1/3] ocfs2: add ocfs2_try_rw_lock and ocfs2_try_inode_lock

[Gang He]

Hi Changwei,


>>> 
> Hi Gang,
> 
> On 2017/11/30 10:45, Gang He wrote:
>> Hello Changwei,
>> 
>> 
>>>>>
>>> On 2017/11/29 16:38, Gang He wrote:
>>>> Add ocfs2_try_rw_lock and ocfs2_try_inode_lock functions, which
>>>> will be used in non-block IO scenarios.
>>>>
>>>> Signed-off-by: Gang He <ghe@xxxxxxxx>
>>>> ---
>>>>    fs/ocfs2/dlmglue.c | 21 +++++++++++++++++++++
>>>>    fs/ocfs2/dlmglue.h |  4 ++++
>>>>    2 files changed, 25 insertions(+)
>>>>
>>>> diff --git a/fs/ocfs2/dlmglue.c b/fs/ocfs2/dlmglue.c
>>>> index 4689940..a68efa3 100644
>>>> --- a/fs/ocfs2/dlmglue.c
>>>> +++ b/fs/ocfs2/dlmglue.c
>>>> @@ -1742,6 +1742,27 @@ int ocfs2_rw_lock(struct inode *inode, int write)
>>>>    	return status;
>>>>    }
>>>>    
>>>> +int ocfs2_try_rw_lock(struct inode *inode, int write)
>>>> +{
>>>> +	int status, level;
>>>> +	struct ocfs2_lock_res *lockres;
>>>> +	struct ocfs2_super *osb = OCFS2_SB(inode->i_sb);
>>>> +
>>>> +	mlog(0, "inode %llu try to take %s RW lock\n",
>>>> +	     (unsigned long long)OCFS2_I(inode)->ip_blkno,
>>>> +	     write ? "EXMODE" : "PRMODE");
>>>> +
>>>> +	if (ocfs2_mount_local(osb))
>>>> +		return 0;
>>>> +
>>>> +	lockres = &OCFS2_I(inode)->ip_rw_lockres;
>>>> +
>>>> +	level = write ? DLM_LOCK_EX : DLM_LOCK_PR;
>>>> +
>>>> +	status = ocfs2_cluster_lock(osb, lockres, level, DLM_LKF_NOQUEUE, 0);
>>>
>>> Hi Gang,
>>> Should we consider about passing a flag - OCFS2_LOCK_NONBLOCK to
>>> ocfs2_cluster_lock. Otherwise a cluster locking progress may be waiting
>>> for accomplishment of DC, which I think violates _NO_WAIT_ semantics.
>> 
>> If ocfs2 is a local file system, we should not wait for any condition, but 
> for a cluster file system,
>> we cannot avoid this totally according to the current DLM lock design, we 
> need to wait for a little to get lock for the first time.
>> Why do we not use OCFS2_LOCK_NONBLOCK flag to get a lock?
>> since this flag is not stable to get a lock no matter this lock is occupied 
> by other nodes, or not.
> I suppose local node must be granted under the condition that it is 
> marked with *OCFS2_LOCK_BLOCKED*. And the control flag 
> _OCFS2_LOCK_NONBLOCK_ will make lock progress directly return -EAGAIN 
> without any waiting.
> 
>> If you use OCFS2_LOCK_NONBLOCK flag to get a fresh lock, you possibly fail 
> or success, depends on when the lock acquisition callback happens.
>> So, I think DLM_LKF_NOQUEUE flag should be more matched to _NO_WAIT_ 
> semantics.
> 
> I thinks OCFS2_LOCK_NONBLOCK doesn't conflict DLM_LKF_NOQUEUE, they are 
> the forth and fifth argument respectively.
Two arguments doesn't conflict, 
but the _OCFS2_LOCK_NONBLOCK_ flag will bring the uncertainty of lock acquisition, since depends on when the lock acquisition callback happens.
In the other words, you possibly do not get a lock for one time if passing the _OCFS2_LOCK_NONBLOCK_ flag.
I think that this flag is suitable to ocfs2_inode_lock_with_page case, since when we can not get the lock immediately, we can try it again and again.
In actually, introducing this flag is to avoid a dead-lock case, a very tricky case. 

Thanks
Gang 


> 
>> we always get a fresh lock successfully, always failed if the lock is/was 
> occupied by other nodes.
> 
> What do you mean by a fresh lock? A lock that is never granted or 
> acquired? If a lock is marked with OCFS2_LOCK_BLOCKED, local node must 
> has acquired it.
> 
> Thanks,
> Changwei
> 
>> This flag can give us a consistent locking behavior.
>> 
>> Thanks
>> Gang
>> 
>> 
>> 
>>>
>>> Thanks,
>>> Changwei.
>>>
>>>> +	return status;
>>>> +}
>>>> +
>>>>    void ocfs2_rw_unlock(struct inode *inode, int write)
>>>>    {
>>>>    	int level = write ? DLM_LOCK_EX : DLM_LOCK_PR;
>>>> diff --git a/fs/ocfs2/dlmglue.h b/fs/ocfs2/dlmglue.h
>>>> index a7fc18b..05910fc 100644
>>>> --- a/fs/ocfs2/dlmglue.h
>>>> +++ b/fs/ocfs2/dlmglue.h
>>>> @@ -116,6 +116,7 @@ void ocfs2_refcount_lock_res_init(struct ocfs2_lock_res
>>> *lockres,
>>>>    int ocfs2_create_new_inode_locks(struct inode *inode);
>>>>    int ocfs2_drop_inode_locks(struct inode *inode);
>>>>    int ocfs2_rw_lock(struct inode *inode, int write);
>>>> +int ocfs2_try_rw_lock(struct inode *inode, int write);
>>>>    void ocfs2_rw_unlock(struct inode *inode, int write);
>>>>    int ocfs2_open_lock(struct inode *inode);
>>>>    int ocfs2_try_open_lock(struct inode *inode, int write);
>>>> @@ -140,6 +141,9 @@ int ocfs2_inode_lock_with_page(struct inode *inode,
>>>>    /* 99% of the time we don't want to supply any additional flags --
>>>>     * those are for very specific cases only. */
>>>>    #define ocfs2_inode_lock(i, b, e) ocfs2_inode_lock_full_nested(i, b, e, 
> 0,
>>> OI_LS_NORMAL)
>>>> +#define ocfs2_try_inode_lock(i, b, e)\
>>>> +		ocfs2_inode_lock_full_nested(i, b, e, OCFS2_META_LOCK_NOQUEUE,\
>>>> +		OI_LS_NORMAL)
>>>>    void ocfs2_inode_unlock(struct inode *inode,
>>>>    		       int ex);
>>>>    int ocfs2_super_lock(struct ocfs2_super *osb,
>>>>
>> 
>>