[PATCHES] Convert gencache to dbwrap to enable mutexes

[Stefan (metze) Metzmacher]

Am 25.06.2014 13:18, schrieb Michael Adam:
> On 2014-06-25 at 08:45 +0200, Stefan (metze) Metzmacher wrote:
>> Am 24.06.2014 22:22, schrieb Christof Schmitt:
>>> On Tue, Jun 24, 2014 at 12:43:54PM -0700, Christof Schmitt wrote:
>>>> On Sat, Jun 21, 2014 at 10:20:44AM +0200, Stefan (metze) Metzmacher wrote:
>>>>> Hi Christof,
>>>>>
>>>>>>> While having a look at it I may found a problem with mutexes and ctdb.
>>>>>>>
>>>>>>>         /* only pass through specific flags */
>>>>>>>         tdb_flags &= TDB_SEQNUM|TDB_VOLATILE;
>>>>>>>
>>>>>>> in db_open_ctdb() seems to completely useless (or wrong).
>>>>>>> ctdbd_db_attach() is called before and gets the raw value of tdb_flags.
>>>>>>> It has only has effect for
>>>>>>>
>>>>>>>         db_ctdb->wtdb = tdb_wrap_open(db_ctdb, db_path, hash_size,
>>>>>>>                                       lpcfg_tdb_flags(lp_ctx, tdb_flags),
>>>>>>>                                       O_RDWR, 0);
>>>>>>>
>>>>>>> So I think we can just remove tdb_flags &= TDB_SEQNUM|TDB_VOLATILE; above
>>>>>>> and let lpcfg_tdb_flags() remove TDB_MUTEX_LOCKING when adding TDB_NOMMAP.
>>>>>>
>>>>>> The two patches at the end of the changeset implement those changes.
>>>>>
>>>>> Thanks! I'll try an autobuild on a machine with mutex support next week.
>>>>
>>>> I just discovered another issue. gencache opens databases with CLEAR_IF_FIRST.
>>>> That is not allowed with mutexes and it also triggers a warning in dbwrap when
>>>> transactions are being used.
>>>>
>>>> The easiest option would be using dbwrap only for gencache_notrans.tdb and
>>>> removing CLEAR_IF_FIRST for that database. For gencache.tdb, i am not sure how
>>>> to handle the case when tdb_check fails without CLEAR_IF_FIRST.
>>>
>>> Sorry for the confusion. CLEAR_IF_FIRST has to be set for mutexes. The
>>> main issue is that transactions are not allowed in dbwrap if
>>> CLEAR_IF_FIRST is set. My idea here is to adapt dwrap to only warn if
>>> the database has been opened through db_open which allows the clustered
>>> usage. Databases opened directly with dbwrap_local_open should still
>>> allow transactions and CLEAR_IF_FIRST.
>>
>> Transactions are not allowed with mutexes if I remember correctly.
> 
> Generally, we try to implement the pattern that a
> database with CLEAR_IF_FIRST does not use transactions.
> For the gencache case, a transaction is used on gencache_notrans
> (despite the name...:-) only in the stabilize operation that
> stabilizes the contents throught to the persistent gencache.tdb.
> It currently works like this:
> 
> transaction_start(cache)
> transaction_start(cache_notrans)
> traverse(stabilize_fn)
> transaction_commit(cache)
> transaction_commit(cache_notrans)
> 
> where stabilize_fn does this on a record:
> 
> 1. store it to or delete it from cache
>    (depending on the timeout)
> 2. delete it from the cache_notrans
> 
> I'll think we should avoid the transaction here on
> gencache_notrans, too, and I will provide a patch that
> does it like this:
> 
> tdb_allrecord_lock(cache_notrans)
> transaction_start(cache)
> traverse(stabilize_fn_mod)
> tdb_wipe_all(cache_notrans)
> transaction_commit(cache)

I think we should do the commit before wipe_all

> tdb_allrecord_unloc(cache_notrans)

metze

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