From: Gang He <ghe@suse.com>
To: Joseph Qi <joseph.qi@linux.alibaba.com>,
mark@fasheh.com, jlbec@evilplan.org
Cc: linux-kernel@vger.kernel.org, ocfs2-devel@oss.oracle.com
Subject: Re: [Ocfs2-devel] [PATCH] ocfs2: ocfs2_downconvert_lock failure results in deadlock
Date: Fri, 27 Aug 2021 13:50:32 +0800 [thread overview]
Message-ID: <f19462cc-b816-ac40-e86a-a1900611c371@suse.com> (raw)
In-Reply-To: <0a8db668-b6f4-4b33-086a-2b522b343cf1@linux.alibaba.com>
On 2021/8/26 17:55, Joseph Qi wrote:
>
>
> 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@suse.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.
OK
>
>>
>>>
>>>> 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;
> }
According to the current code, it is easy to
goto leave label with set ctl->requeue = 1 for requeue;
Thanks
Gang
>
> ...
> requeue:
> ctl->requeue = 1;
> return 0;
>
>
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prev parent reply other threads:[~2021-08-27 5:51 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-26 6:10 [Ocfs2-devel] [PATCH] ocfs2: ocfs2_downconvert_lock failure results in deadlock Gang He
2021-08-26 8:23 ` Joseph Qi
2021-08-26 8:45 ` Gang He
2021-08-26 9:55 ` Joseph Qi
2021-08-27 5:50 ` Gang He [this message]
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