Hi Andrew,
On 2015/4/29 4:32, Andrew Morton wrote:
> On Sat, 25 Apr 2015 15:05:15 +0800 Joseph Qi <joseph.qi@xxxxxxxxxx> 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.
> .
>
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