[Ocfs2-devel] [PATCH] ocfs2/dlm: remove potential deadlock

[Tao Ma <tao.ma@oracle.com>]

hi wengang,

On 06/21/2010 01:31 PM, Wengang Wang wrote:
> Why atomic operations on dlm_refs need spinlock's protect?
>
>          /* NOTE: Next three are protected by dlm_domain_lock */
>          struct kref dlm_refs;
>          enum dlm_ctxt_state dlm_state;
>          unsigned int num_joins;
I don't dive into the code, so if there is something wrong, please 
correct me.
AFAICS, kref_put isn't an atomic operation, it need to call release to 
free the container of kref.

Regards,
Tao
>
> regards,
> wengang.
>
> On 10-06-16 14:52, Wengang Wang wrote:
>> When we need to take both dlm_domain_lock and dlm->spinlock, we should take
>> them in order of:
>> dlm_domain_lock then dlm->spinlock.
>>
>> There is pathes disobey this order. That is calling dlm_lockres_put with dlm->
>> spinlock held. dlm_lockres_put() finally calls dlm_put() which take
>> dlm_domain_lock.
>>
>> The fix is moving the locking on dlm_domain_lock to dlm_ctxt_release() from
>> dlm_put(). dlm_ctxt_release() is only called on the release of the last
>> reference. Any path should not be holding dlm->spinlock when dropping the "last"
>> reference.
>>
>> Signed-off-by: Wengang Wang<wen.gang.wang@oracle.com>
>> ---
>>   fs/ocfs2/dlm/dlmdomain.c |   12 ++++--------
>>   1 files changed, 4 insertions(+), 8 deletions(-)
>>
>> diff --git a/fs/ocfs2/dlm/dlmdomain.c b/fs/ocfs2/dlm/dlmdomain.c
>> index ab82add..754baf2 100644
>> --- a/fs/ocfs2/dlm/dlmdomain.c
>> +++ b/fs/ocfs2/dlm/dlmdomain.c
>> @@ -321,28 +321,24 @@ static void dlm_ctxt_release(struct kref *kref)
>>
>>   	dlm = container_of(kref, struct dlm_ctxt, dlm_refs);
>>
>> +	if (spin_is_locked(&dlm->spinlock))
>> +		BUG();
>>   	BUG_ON(dlm->num_joins);
>>   	BUG_ON(dlm->dlm_state == DLM_CTXT_JOINED);
>>
>> +	spin_lock(&dlm_domain_lock);
>>   	/* we may still be in the list if we hit an error during join. */
>>   	list_del_init(&dlm->list);
>> -
>>   	spin_unlock(&dlm_domain_lock);
>>
>> -	mlog(0, "freeing memory from domain %s\n", dlm->name);
>> -
>>   	wake_up(&dlm_domain_events);
>> -
>> +	mlog(0, "freeing memory from domain %s\n", dlm->name);
>>   	dlm_free_ctxt_mem(dlm);
>> -
>> -	spin_lock(&dlm_domain_lock);
>>   }
>>
>>   void dlm_put(struct dlm_ctxt *dlm)
>>   {
>> -	spin_lock(&dlm_domain_lock);
>>   	kref_put(&dlm->dlm_refs, dlm_ctxt_release);
>> -	spin_unlock(&dlm_domain_lock);
>>   }
>>
>>   static void __dlm_get(struct dlm_ctxt *dlm)
>> --
>> 1.6.6.1
>>
>>
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>
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