[Gluster-devel] Regarding doing away with refkeeper in locks xlator
Pranith Kumar Karampuri
pkarampu at redhat.com
Sun Jun 8 17:42:00 UTC 2014
On 06/06/2014 11:38 AM, Pranith Kumar Karampuri wrote:
>
> On 06/06/2014 11:37 AM, Anand Avati wrote:
>>
>>
>>
>> On Thu, Jun 5, 2014 at 10:56 PM, Pranith Kumar Karampuri
>> <pkarampu at redhat.com <mailto:pkarampu at redhat.com>> wrote:
>>
>>
>> On 06/06/2014 10:47 AM, Pranith Kumar Karampuri wrote:
>>>
>>> On 06/06/2014 10:43 AM, Anand Avati wrote:
>>>>
>>>>
>>>>
>>>> On Thu, Jun 5, 2014 at 9:54 PM, Pranith Kumar Karampuri
>>>> <pkarampu at redhat.com <mailto:pkarampu at redhat.com>> wrote:
>>>>
>>>>
>>>> On 06/06/2014 10:02 AM, Anand Avati wrote:
>>>>> On Thu, Jun 5, 2014 at 7:52 PM, Pranith Kumar Karampuri
>>>>> <pkarampu at redhat.com <mailto:pkarampu at redhat.com>> wrote:
>>>>>
>>>>>>
>>>>>> This sounds a bit complicated. I think there is a
>>>>>> much simpler solution:
>>>>>>
>>>>>> - First, make update_refkeeper() check for blocked
>>>>>> locks (which I mentioned as "optional" previously)
>>>>>>
>>>>>> - Make grant_blocked_locks() double up and do the job
>>>>>> of update_refkeeper() internally.
>>>>>>
>>>>>> Something which looks like this:
>>>>>>
>>>>>> diff --git a/xlators/features/locks/src/common.c
>>>>>> b/xlators/features/locks/src/common.c
>>>>>> index f6c71c1..38df385 100644
>>>>>> --- a/xlators/features/locks/src/common.c
>>>>>> +++ b/xlators/features/locks/src/common.c
>>>>>> @@ -126,8 +126,14 @@ __pl_inode_is_empty (pl_inode_t
>>>>>> *pl_inode)
>>>>>> if (!list_empty (&dom->entrylk_list))
>>>>>> is_empty = 0;
>>>>>> + if (!list_empty (&dom->blocked_entrylks))
>>>>>> + is_empty = 0;
>>>>>> +
>>>>>> if (!list_empty (&dom->inodelk_list))
>>>>>> is_empty = 0;
>>>>>> +
>>>>>> + if (!list_empty (&dom->blocked_inodelks))
>>>>>> + is_empty = 0;
>>>>>> }
>>>>>> return is_empty;
>>>>>> @@ -944,12 +950,18 @@ grant_blocked_locks (xlator_t
>>>>>> *this, pl_inode_t *pl_inode)
>>>>>> struct list_head granted_list;
>>>>>> posix_lock_t *tmp = NULL;
>>>>>> posix_lock_t *lock = NULL;
>>>>>> + inode_t *unref = NULL;
>>>>>> INIT_LIST_HEAD (&granted_list);
>>>>>> pthread_mutex_lock (&pl_inode->mutex);
>>>>>> {
>>>>>> __grant_blocked_locks (this, pl_inode, &granted_list);
>>>>>> +
>>>>>> + if (__pl_inode_is_empty (pl_inode) &&
>>>>>> pl_inode->refkeeper) {
>>>>>> + unref = pl_inode->refkeeper;
>>>>>> + pl_inode->refkeeper = NULL;
>>>>>> + }
>>>>>> }
>>>>>> pthread_mutex_unlock (&pl_inode->mutex);
>>>>>> @@ -965,6 +977,9 @@ grant_blocked_locks (xlator_t
>>>>>> *this, pl_inode_t *pl_inode)
>>>>>> GF_FREE (lock);
>>>>>> }
>>>>>> + if (unref)
>>>>>> + inode_unref (unref);
>>>>>> +
>>>>>> return;
>>>>>> }
>>>>>>
>>>>>>
>>>>>> This should make pl_disconnect_cbk() pretty much race
>>>>>> free w.r.t refkpeer. Thoughts?
>>>>> Lets say C1 is doing pl_inodelk_client_cleanup. After
>>>>> the second for-loop(All granted and blocked locks are
>>>>> out of the domain) if an unlock on the final granted
>>>>> lock on that inode from client C2 completes, refkeeper
>>>>> would be set to NULL and unrefed leading to zero refs
>>>>> on that inode i.e. pl_forget will also happen. In 3rd
>>>>> for-loop pl_inode is already freed and leads to free'd
>>>>> memory access and will crash.
>>>>>
>>>>>
>>>>>
>>>>> We also need:
>>>>>
>>>>> diff --git a/xlators/features/locks/src/inodelk.c b/xlators/features/locks/src/inodelk.c
>>>>> index c76cb7f..2aceb8a 100644
>>>>> --- a/xlators/features/locks/src/inodelk.c
>>>>> +++ b/xlators/features/locks/src/inodelk.c
>>>>> @@ -494,13 +494,13 @@ pl_inodelk_client_cleanup (xlator_t *this, pl_ctx_t *ctx)
>>>>>
>>>>> dom = get_domain (pl_inode, l->volume);
>>>>>
>>>>> - grant_blocked_inode_locks (this, pl_inode, dom);
>>>>> -
>>>>> pthread_mutex_lock (&pl_inode->mutex);
>>>>> {
>>>>> __pl_inodelk_unref (l);
>>>>> }
>>>>> pthread_mutex_unlock (&pl_inode->mutex);
>>>>> +
>>>>> + grant_blocked_inode_locks (this, pl_inode, dom);
>>>>> }
>>>>>
>>>>> return 0;
>>>>>
>>>>> Missed this in the last patch.
>>>> It still doesn't solve the problem I described earlier. By
>>>> the time it executes this third loop refkeeper is already
>>>> unreffed when C2 unlocks.
>>>>
>>>>
>>>>
>>>> Right, we will need to introduce an "in_cleanup" counter, if
>>>> set pl_update_refkeeper() should not unref. Increment the
>>>> in_cleanup() in the first lookup, and decrement it in the last
>>>> loop, just before calling grant_blocked_locks() (along with the
>>>> patches in my last 2 mails)
>>> s/first lookup/first loop/ ?
>> Consider the following scenario:
>> There are two granted locks L1, L2 from C1, C2 clients
>> respectively on same inode.
>> C1 gets disconnected.
>> C2 issues a unlock.
>>
>> This is the sequence of steps:
>> 1) C1 executes first loop, increments in_cleanup to 1
>> 2) C2 executes pl_inode_setlk and removed L2 from granted list.
>> It is now just before grant_blocked_inode_locks()
>> 3) C1 starts 3rd for loop and unrefs L1, decrements in_cleanup to 0
>> 4) C2 executes grant_blocked_inode_locks() and decrements the
>> refkeepr, sets it to NULL and unwinds. This destroys the inode so
>> pl_inode is freed.
>> 5) C1 calls grant_blocked_inode_locks with pl_inode which is free'd
>>
>>
>> Yeah, we need a version of grant_blocked_inode_locks() which
>> decrements in_cleanup in its locked region.
> I was just thinking the same. I will update you if it works.
Avati,
This is the final implementation of the solution kritika and I
decided upon. No double locks are needed. Most of the code is comments
:-). I guess the patch is less than 50 lines. Let us know your inputs.
http://review.gluster.com/7981
Pranith
>
> Pranith
>
>
>
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