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

[Gang He]

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.
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.
we always get a fresh lock successfully, always failed if the lock is/was occupied by other nodes.
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,
>>