linux-kernel - Re: [PATCH] ocfs2: ocfs2_downconvert

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Date:   Thu, 26 Aug 2021 17:55:47 +0800
From:   Joseph Qi <joseph.qi@...ux.alibaba.com>
To:     Gang He <ghe@...e.com>, mark@...heh.com, jlbec@...lplan.org
Cc:     linux-kernel@...r.kernel.org, ocfs2-devel@....oracle.com,
        akpm@...ux-foundation.org
Subject: Re: [PATCH] ocfs2: ocfs2_downconvert_lock failure results in deadlock



On 8/26/21 4:45 PM, Gang He wrote:
> Hi Joseph,
> 
> On 2021/8/26 16:23, Joseph Qi wrote:
>>
>>
>> On 8/26/21 2:10 PM, Gang He wrote:
>>> Usually, ocfs2_downconvert_lock() function always downconverts
>>> dlm lock to the expected level for satisfy dlm bast requests
>>
>> s/for/to
>>
>>> from the other nodes.
>>> But there is a rare situation. When dlm lock conversion is being
>>> canceled, ocfs2_downconvert_lock() function will return -EBUSY.
>>> You need to be aware that ocfs2_cancel_convert() function is
>>> asynchronous in fsdlm implementation.
>>> If we does not requeue this lockres entry, ocfs2 downconvert
>>> thread no longer handles this dlm lock bast request. Then, the
>>> other nodes will not get the dlm lock again, the current node's
>>> process will be blocked when acquire this dlm lock again.
>>>
>>> Signed-off-by: Gang He <ghe@...e.com>
>>> ---
>>>   fs/ocfs2/dlmglue.c | 13 +++++++++----
>>>   1 file changed, 9 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/fs/ocfs2/dlmglue.c b/fs/ocfs2/dlmglue.c
>>> index 48fd369c29a4..c454c218fbfe 100644
>>> --- a/fs/ocfs2/dlmglue.c
>>> +++ b/fs/ocfs2/dlmglue.c
>>> @@ -3671,13 +3671,11 @@ static int ocfs2_downconvert_lock(struct ocfs2_super *osb,
>>>                    OCFS2_LOCK_ID_MAX_LEN - 1);
>>>       lockres_clear_pending(lockres, generation, osb);
>>>       if (ret) {
>>> -        ocfs2_log_dlm_error("ocfs2_dlm_lock", ret, lockres);
>>> +        if (ret != -EBUSY)
>>> +            ocfs2_log_dlm_error("ocfs2_dlm_lock", ret, lockres);
>>
>> Do we have to treat EBUSY as a normal case?
> Yes, this return code is expected when call dlm_lock() to convert a dlm lock to another level, but this dlm lock is being cancelled.
> As I said in another mail, for fsdlm implementation,ocfs2_cancel_convert
> will return immediately, but the related dlm lock will(is) be cancelled in background. For o2dlm implementation,ocfs2_cancel_convert will return after the dlm lock is cancelled and it's ast is invoked, that is why o2cb based ocfs2 does not encounter -EBUSY return code in my test script, of course, this kind of implementation will block other lockres entries to be handled for a little time in down-convert thread.

Better to leave this log for later issue tracking.
I'm worrying about if there are other cases here.

> 
>>
>>>           ocfs2_recover_from_dlm_error(lockres, 1);
>>> -        goto bail;
>>>       }
>>>   -    ret = 0;
>>> -bail:
>>>       return ret;
>>>   }
>>>   @@ -3912,6 +3910,13 @@ static int ocfs2_unblock_lock(struct ocfs2_super *osb,
>>>       spin_unlock_irqrestore(&lockres->l_lock, flags);
>>>       ret = ocfs2_downconvert_lock(osb, lockres, new_level, set_lvb,
>>>                        gen);
>>> +    /* ocfs2_cancel_convert() is in progress, try again later */
>>> +    if (ret == -EBUSY) {
>>> +        ctl->requeue = 1;
>>> +        mlog(ML_BASTS, "lockres %s, ReQ: Downconvert busy\n",
>>> +             lockres->l_name);
>>> +        ret = 0;
>>
>> Ditto. If EBUSY is not a normal case, I'd like just requeue it but not
>> change it to normal return code.
>> You know ML_BASTS is always switched off in production environment.
> Since this case should be considered as a normal case, although it's rare.
> We should not print any error message to kernel journal, but if the user
> turn on the BASTS trace, he should watch this trace for debugging.
> 

Okay, since we leave an error message above, we can return normal to
caller. And now caller only print a simple error which doesn't have
much meaning.

BTW, could we change it like:

ret = ocfs2_downconvert_lock();
if (ret == -EBUSY) {
	mlog(ML_BASTS, ...);
	/* Describe the case why we have to requeue */
	goto requeue;
}

...
requeue:
	ctl->requeue = 1;
	return 0;