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Subject: Re: [PATCH] ocfs2: ocfs2_downconvert_lock failure results in deadlock
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,
        akpm@linux-foundation.org
References: <20210826061038.22295-1-ghe@suse.com>
 <b3f498eb-7de5-8a7f-0f52-01c1e2caa5d8@linux.alibaba.com>
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 <0a8db668-b6f4-4b33-086a-2b522b343cf1@linux.alibaba.com>
From:   Gang He <ghe@suse.com>
Message-ID: <f19462cc-b816-ac40-e86a-a1900611c371@suse.com>
Date:   Fri, 27 Aug 2021 13:50:32 +0800
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On 2021/8/26 17:55, Joseph Qi wrote:
>=20
>=20
> 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>
>>>> ---
>>>>  =C2=A0 fs/ocfs2/dlmglue.c | 13 +++++++++----
>>>>  =C2=A0 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,
>>>>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 OCFS2_LOCK_ID_MAX_LEN - 1);
>>>>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 lockres_clear_pending(lockres, generat=
ion, osb);
>>>>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (ret) {
>>>> -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 ocfs2_log_dlm_error("ocfs2=
_dlm_lock", ret, lockres);
>>>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (ret !=3D -EBUSY)
>>>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 oc=
fs2_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 af=
ter the dlm lock is cancelled and it's ast is invoked, that is why o2cb bas=
ed ocfs2 does not encounter -EBUSY return code in my test script, of course=
, this kind of implementation will block other lockres entries to be handle=
d for a little time in down-convert thread.
>=20
> Better to leave this log for later issue tracking.
> I'm worrying about if there are other cases here.
OK

>=20
>>
>>>
>>>>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 ocfs2_recover_=
from_dlm_error(lockres, 1);
>>>> -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 goto bail;
>>>>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 }
>>>>  =C2=A0 -=C2=A0=C2=A0=C2=A0 ret =3D 0;
>>>> -bail:
>>>>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 return ret;
>>>>  =C2=A0 }
>>>>  =C2=A0 @@ -3912,6 +3910,13 @@ static int ocfs2_unblock_lock(struct oc=
fs2_super *osb,
>>>>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 spin_unlock_irqrestore(&lockres->l_loc=
k, flags);
>>>>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 ret =3D ocfs2_downconvert_lock(osb, lo=
ckres, new_level, set_lvb,
>>>>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 gen);
>>>> +=C2=A0=C2=A0=C2=A0 /* ocfs2_cancel_convert() is in progress, try agai=
n later */
>>>> +=C2=A0=C2=A0=C2=A0 if (ret =3D=3D -EBUSY) {
>>>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 ctl->requeue =3D 1;
>>>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 mlog(ML_BASTS, "lockres %s=
, ReQ: Downconvert busy\n",
>>>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0 lockres->l_name);
>>>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 ret =3D 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 rar=
e.
>> 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.
>>
>=20
> 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.
>=20
> BTW, could we change it like:
>=20
> ret =3D ocfs2_downconvert_lock();
> if (ret =3D=3D -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 =3D 1 for requeue;


Thanks
Gang

>=20
> ...
> requeue:
> 	ctl->requeue =3D 1;
> 	return 0;
>=20
>=20