[Ocfs2-devel] [PATCH] ocfs2/dlm: fix race between purge and get lock resource

[Junxiao Bi]

On 04/29/2015 08:51 AM, Joseph Qi wrote:
> Hi Andrew,
>
> On 2015/4/29 4:32, Andrew Morton wrote:
>> On Sat, 25 Apr 2015 15:05:15 +0800 Joseph Qi <joseph.qi at huawei.com> wrote:
>>
>>> There is a race between purge and get lock resource, which will lead to
>>> ast unfinished and system hung. The case is described below:
>>>
>>> mkdir                                  dlm_thread
>>> -----------------------------------------------------------------------
>>> o2cb_dlm_lock                        |
>>> -> dlmlock                           |
>>>    -> dlm_get_lock_resource           |
>>>      -> __dlm_lookup_lockres_full     |
>>>        -> spin_unlock(&dlm->spinlock) |
>>>                                       | dlm_run_purge_list
>>>                                       | -> dlm_purge_lockres
>>>                                       |   -> dlm_drop_lockres_ref
>>>                                       |   -> spin_lock(&dlm->spinlock)
>>>                                       |   -> spin_lock(&res->spinlock)
>>>                                       |   -> ~DLM_LOCK_RES_DROPPING_REF
>>>                                       |   -> spin_unlock(&res->spinlock)
>>>                                       |   -> spin_unlock(&dlm->spinlock)
>>>        -> spin_lock(&tmpres->spinlock)|
>>>        DLM_LOCK_RES_DROPPING_REF cleared |
>>>        -> spin_unlock(&tmpres->spinlock) |
>>>        return the purged lockres         |
>>>
>>> So after this, once ast comes, it will ingore the ast because the
>>> lockres cannot be found anymore. Thus the OCFS2_LOCK_BUSY won't be
>>> cleared and corresponding thread hangs.
>>> The &dlm->spinlock was hold when checking DLM_LOCK_RES_DROPPING_REF at
>>> the very begining. And commit 7b791d6856 (ocfs2/dlm: Fix race during
>>> lockres mastery) moved it up because of the possible wait.
>>> So take the &dlm->spinlock and introduce a new wait function to fix the
>>> race.
>>>
>>> ...
>>>
>>> --- a/fs/ocfs2/dlm/dlmthread.c
>>> +++ b/fs/ocfs2/dlm/dlmthread.c
>>> @@ -77,6 +77,29 @@ repeat:
>>>   	__set_current_state(TASK_RUNNING);
>>>   }
>>>
>>> +void __dlm_wait_on_lockres_flags_new(struct dlm_ctxt *dlm,
>>> +		struct dlm_lock_resource *res, int flags)
>>> +{
>>> +	DECLARE_WAITQUEUE(wait, current);
>>> +
>>> +	assert_spin_locked(&dlm->spinlock);
>>> +	assert_spin_locked(&res->spinlock);
>> Not strictly needed - lockdep will catch this.   A minor thing.
>>
>>> +	add_wait_queue(&res->wq, &wait);
>>> +repeat:
>>> +	set_current_state(TASK_UNINTERRUPTIBLE);
>>> +	if (res->state & flags) {
>>> +		spin_unlock(&res->spinlock);
>>> +		spin_unlock(&dlm->spinlock);
>>> +		schedule();
>>> +		spin_lock(&dlm->spinlock);
>>> +		spin_lock(&res->spinlock);
>>> +		goto repeat;
>>> +	}
>>> +	remove_wait_queue(&res->wq, &wait);
>>> +	__set_current_state(TASK_RUNNING);
>>> +}
>> This is pretty nasty.  Theoretically this could spin forever, if other
>> tasks are setting the flag in a suitably synchronized fashion.
>>
>> Is there no clean approach?  A reorganization of the locking?
>>
> Do you mean the flag won't be cleared forever? If so, only taking
> &res->spinlock also has the same risk. But we haven't found this in our
> test/production environments so far.
> To fix the race case above, I don't have another approach besides taking
> &dlm->spinlock.
Sorry, just found this patch was sent twice, i reply on the first one. 
This fix looks complicated. I posted another simpler fix on old thread.

Thanks,
Junxiao.
>> .
>>
>
>
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