[Ocfs2-devel] [PATCH] ocfs2: do not set OCFS2_LOCK_UPCONVERT_FINISHING if nonblocking lock can not be granted at once

[Junxiao Bi]

On 11/07/2014 02:36 PM, Xue jiufei wrote:
> Hi Junxiao
> 
> On 2014/11/7 13:54, Junxiao Bi wrote:
>> Hi Jiufei,
>>
>> On 11/07/2014 11:41 AM, Xue jiufei wrote:
>>> Function ocfs2_readpages() use nonblocking flag to avoid page lock
>>> inversion. It will trigger cluster hang because that flag
>>> OCFS2_LOCK_UPCONVERT_FINISHING is not cleared if nonblocking lock
>>> cannot be granted at once. The flag would prevent dc thread from
>>> downconverting. So other nodes cannot acheive this lockres for ever.
>>>
>>> So we should not set OCFS2_LOCK_UPCONVERT_FINISHING when receiving ast
>>> if nonblocking lock had already returned.
>>>
>>
>> Use OCFS2_LOCK_NONBLOCK_FINISHED can't fix this issue. Like there are
>> two processes doing non-blocking lock in the following order.
> I am wondering why these two processes can lock in such order.

Yes, you are right. LOCK_BUSY flag stop this race. Your fix works. But
just as i mentioned, i think for non-blocking lock, no need do cluster
wild locking, because AST is async, before it reached, most of the time,
the non-blocking lock had returned fail.

Thanks,
Junxiao.
>> 1. Process A issue non-blocking lock fail, set OCFS2_LOCK_NONBLOCK_FINISHED.
> We had set OCFS2_LOCK_BUSY flag before process A call ocfs2_dlm_lock(),
>> 2. Process B issue non-blocking lock fail, set
>> OCFS2_LOCK_NONBLOCK_FINISHED again.
> So process B must wait until ast for Process A returns and clear OCFS2_LOCK_BUSY.
> 
>> 3. Ast for process A come, clear OCFS2_LOCK_NONBLOCK_FINISHED.
>> 4. Ast for process B come, set OCFS2_LOCK_UPCONVERT_FINISHING.
>>
>> So down convert can still be blocked.
>>
>> To fix this issue, I think no need to call ocfs2_dlm_lock() for
>> non-blocking lock, just check local lockres->l_level is enough, if
>> request level equal or smaller than it, lock success, or lock fail.
>>
>> Thanks,
>> Junxiao.
>>
> Thanks,
> jiufei.
> 
>>> Signed-off-by: joyce.xue <xuejiufei at huawei.com>
>>> ---
>>>  fs/ocfs2/dlmglue.c | 37 +++++++++++++++++++++++++++++++------
>>>  fs/ocfs2/ocfs2.h   |  6 ++++++
>>>  2 files changed, 37 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/fs/ocfs2/dlmglue.c b/fs/ocfs2/dlmglue.c
>>> index 21262f2..1ddb6f1 100644
>>> --- a/fs/ocfs2/dlmglue.c
>>> +++ b/fs/ocfs2/dlmglue.c
>>> @@ -861,8 +861,13 @@ static inline void ocfs2_generic_handle_convert_action(struct ocfs2_lock_res *lo
>>>  	 * We set the OCFS2_LOCK_UPCONVERT_FINISHING flag before clearing
>>>  	 * the OCFS2_LOCK_BUSY flag to prevent the dc thread from
>>>  	 * downconverting the lock before the upconvert has fully completed.
>>> +	 * Do not prevent the dc thread from downconverting if NONBLOCK lock
>>> +	 * had already returned.
>>>  	 */
>>> -	lockres_or_flags(lockres, OCFS2_LOCK_UPCONVERT_FINISHING);
>>> +	if (!(lockres->l_flags & OCFS2_LOCK_NONBLOCK_FINISHED))
>>> +		lockres_or_flags(lockres, OCFS2_LOCK_UPCONVERT_FINISHING);
>>> +	else
>>> +		lockres_clear_flags(lockres, OCFS2_LOCK_NONBLOCK_FINISHED);
>>>  
>>>  	lockres_clear_flags(lockres, OCFS2_LOCK_BUSY);
>>>  }
>>> @@ -1324,13 +1329,12 @@ static void lockres_add_mask_waiter(struct ocfs2_lock_res *lockres,
>>>  
>>>  /* returns 0 if the mw that was removed was already satisfied, -EBUSY
>>>   * if the mask still hadn't reached its goal */
>>> -static int lockres_remove_mask_waiter(struct ocfs2_lock_res *lockres,
>>> +static int __lockres_remove_mask_waiter(struct ocfs2_lock_res *lockres,
>>>  				      struct ocfs2_mask_waiter *mw)
>>>  {
>>> -	unsigned long flags;
>>>  	int ret = 0;
>>>  
>>> -	spin_lock_irqsave(&lockres->l_lock, flags);
>>> +	assert_spin_locked(&lockres->l_lock);
>>>  	if (!list_empty(&mw->mw_item)) {
>>>  		if ((lockres->l_flags & mw->mw_mask) != mw->mw_goal)
>>>  			ret = -EBUSY;
>>> @@ -1338,6 +1342,18 @@ static int lockres_remove_mask_waiter(struct ocfs2_lock_res *lockres,
>>>  		list_del_init(&mw->mw_item);
>>>  		init_completion(&mw->mw_complete);
>>>  	}
>>> +
>>> +	return ret;
>>> +}
>>> +
>>> +static int lockres_remove_mask_waiter(struct ocfs2_lock_res *lockres,
>>> +				      struct ocfs2_mask_waiter *mw)
>>> +{
>>> +	unsigned long flags;
>>> +	int ret = 0;
>>> +
>>> +	spin_lock_irqsave(&lockres->l_lock, flags);
>>> +	ret = __lockres_remove_mask_waiter(lockres, mw);
>>>  	spin_unlock_irqrestore(&lockres->l_lock, flags);
>>>  
>>>  	return ret;
>>> @@ -1373,6 +1389,7 @@ static int __ocfs2_cluster_lock(struct ocfs2_super *osb,
>>>  	unsigned long flags;
>>>  	unsigned int gen;
>>>  	int noqueue_attempted = 0;
>>> +	int dlm_locked = 0;
>>>  
>>>  	ocfs2_init_mask_waiter(&mw);
>>>  
>>> @@ -1481,6 +1498,7 @@ again:
>>>  			ocfs2_recover_from_dlm_error(lockres, 1);
>>>  			goto out;
>>>  		}
>>> +		dlm_locked = 1;
>>>  
>>>  		mlog(0, "lock %s, successful return from ocfs2_dlm_lock\n",
>>>  		     lockres->l_name);
>>> @@ -1514,10 +1532,17 @@ out:
>>>  	if (wait && arg_flags & OCFS2_LOCK_NONBLOCK &&
>>>  	    mw.mw_mask & (OCFS2_LOCK_BUSY|OCFS2_LOCK_BLOCKED)) {
>>>  		wait = 0;
>>> -		if (lockres_remove_mask_waiter(lockres, &mw))
>>> +		spin_lock_irqsave(&lockres->l_lock, flags);
>>> +		if (__lockres_remove_mask_waiter(lockres, &mw)) {
>>> +			if (dlm_locked)
>>> +				lockres_or_flags(lockres,
>>> +					OCFS2_LOCK_NONBLOCK_FINISHED);
>>> +			spin_unlock_irqrestore(&lockres->l_lock, flags);
>>>  			ret = -EAGAIN;
>>> -		else
>>> +		} else {
>>> +			spin_unlock_irqrestore(&lockres->l_lock, flags);
>>>  			goto again;
>>> +		}
>>>  	}
>>>  	if (wait) {
>>>  		ret = ocfs2_wait_for_mask(&mw);
>>> diff --git a/fs/ocfs2/ocfs2.h b/fs/ocfs2/ocfs2.h
>>> index bbec539..7d6b7d0 100644
>>> --- a/fs/ocfs2/ocfs2.h
>>> +++ b/fs/ocfs2/ocfs2.h
>>> @@ -144,6 +144,12 @@ enum ocfs2_unlock_action {
>>>  						     * before the upconvert
>>>  						     * has completed */
>>>  
>>> +#define OCFS2_LOCK_NONBLOCK_FINISHED (0x00001000) /* NONBLOCK cluster
>>> +						   * lock has already
>>> +						   * returned, do not block
>>> +						   * dc thread from
>>> +						   * downconverting */
>>> +
>>>  struct ocfs2_lock_res_ops;
>>>  
>>>  typedef void (*ocfs2_lock_callback)(int status, unsigned long data);
>>>
>>
>> .
>>
> 
>