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From: Vladislav Shpilevoy via Tarantool-patches <tarantool-patches@dev.tarantool.org>
To: Serge Petrenko <sergepetrenko@tarantool.org>,
	Cyrill Gorcunov <gorcunov@gmail.com>
Cc: tarantool-patches@dev.tarantool.org
Subject: Re: [Tarantool-patches] [PATCH v2 2/2] box: fix an assertion failure in box.ctl.promote()
Date: Wed, 26 May 2021 20:46:20 +0200	[thread overview]
Message-ID: <3552b10e-370d-ddbd-11ed-8c3d5310e651@tarantool.org> (raw)
In-Reply-To: <aad0a4b2-e68b-e39b-8b30-709940ebfd1d@tarantool.org>

>>> @@ -1618,14 +1618,29 @@ box_promote(void)
>>>                    txn_limbo.owner_id);
>>>               return -1;
>>>           }
>>> +        if (txn_limbo_is_empty(&txn_limbo)) {
>>> +            wait_lsn = txn_limbo.confirmed_lsn;
>>> +            goto promote;
>>> +        }
>>>       }
>>>   -    /*
>>> -     * promote() is a no-op on the limbo owner, so all the rows
>>> -     * in the limbo must've come through the applier meaning they already
>>> -     * have an lsn assigned, even if their WAL write hasn't finished yet.
>>> -     */
>>> -    wait_lsn = txn_limbo_last_synchro_entry(&txn_limbo)->lsn;
>>> +    struct txn_limbo_entry *last_entry;
>>> +    last_entry = txn_limbo_last_synchro_entry(&txn_limbo);
>>> +    /* Wait for the last entries WAL write. */
>>> +    if (last_entry->lsn < 0) {
>>> +        if (wal_sync(NULL) < 0)
>>> +            return -1;
>>> +        if (txn_limbo_is_empty(&txn_limbo)) {
>>> +            wait_lsn = txn_limbo.confirmed_lsn;
>>> +            goto promote;
>>> +        }
>>> +        if (last_entry != txn_limbo_last_synchro_entry(&txn_limbo)) {
>> This is a bit dangerous. We cache a pointer and then go to fiber_yield,
>> which switches context, at this moment the pointer become dangling one
>> and we simply can't be sure if it _were_ reused. IOW, Serge are we
>> 100% sure that the same pointer with same address but with new data
>> won't appear here as last entry in limbo?
> 
> I agree this solution is not perfect.
> 
> An alternative would be to do the following:
> 1) Check that the limbo owner hasn't changed
> 2) Check that the last entry has positive lsn (e.g. it's not a new entry which
>     wasn't yet written to WAL). And that this lsn is equal to the lsn of our entry.
> 
> But what if our entry was confirmed and destroyed during wal_sync()? We can't compare
> other entries lsn with this ones.

As decided in the chat, you can use txn->id. It is unique until
restart and should help to detect if the last transaction has
changed.

  reply	other threads:[~2021-05-26 18:46 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-25 10:39 [Tarantool-patches] [PATCH v2 0/2] " Serge Petrenko via Tarantool-patches
2021-05-25 10:39 ` [Tarantool-patches] [PATCH v2 1/2] box: refactor in_promote using a guard Serge Petrenko via Tarantool-patches
2021-05-26  7:25   ` Cyrill Gorcunov via Tarantool-patches
2021-05-27 10:57     ` Serge Petrenko via Tarantool-patches
2021-05-27 11:02       ` Cyrill Gorcunov via Tarantool-patches
2021-05-25 10:39 ` [Tarantool-patches] [PATCH v2 2/2] box: fix an assertion failure in box.ctl.promote() Serge Petrenko via Tarantool-patches
2021-05-26  6:14   ` Cyrill Gorcunov via Tarantool-patches
2021-05-26  8:25     ` Serge Petrenko via Tarantool-patches
2021-05-26 18:46       ` Vladislav Shpilevoy via Tarantool-patches [this message]
2021-05-27 10:53         ` Serge Petrenko via Tarantool-patches
2021-05-27 11:03           ` Cyrill Gorcunov via Tarantool-patches
2021-05-27 19:30           ` Vladislav Shpilevoy via Tarantool-patches
2021-06-01 12:20 ` [Tarantool-patches] [PATCH v2 0/2] " Kirill Yukhin via Tarantool-patches

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