linux-ext4 - Re: [PATCH 1/2] Updated locking documentation for transaction

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Message-ID: <ec682a4c-f4f7-35fe-dc35-6c0b53d6ecda@tu-dortmund.de>
Date:   Fri, 26 Mar 2021 09:18:45 +0100
From:   Alexander Lochmann <alexander.lochmann@...dortmund.de>
To:     Jan Kara <jack@...e.cz>
Cc:     Horst Schirmeier <horst.schirmeier@...dortmund.de>,
        "Theodore Ts'o" <tytso@....edu>, Jan Kara <jack@...e.com>,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] Updated locking documentation for transaction_t

On 11.02.21 10:30, Jan Kara wrote:
>> diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
>> index 99d3cd051ac3..18f77d9b1745 100644
>> --- a/include/linux/jbd2.h
>> +++ b/include/linux/jbd2.h
>> @@ -594,18 +594,18 @@ struct transaction_s
>>  	 */
>>  	unsigned long		t_log_start;
>>  
>> -	/* Number of buffers on the t_buffers list [j_list_lock] */
>> +	/* Number of buffers on the t_buffers list [j_list_lock, no lock for quick racy checks] */
>>  	int			t_nr_buffers;
> 
> So this case is actually somewhat different now that I audited the uses.
> There are two types of users - commit code (fs/jbd2/commit.c) and others.
> Other users properly use j_list_lock to access t_nr_buffers. Commit code
> does not use any locks because committing transaction is fully in
> ownership of the jbd2 thread and all other users need to check & wait for
> commit to be finished before doing anything with the transaction's buffers.

I'm still trying understand how thinks work:
Accesses to transaction_t might occur from different contexts. Thus,
locks are necessary. If it comes to the commit phase, every other
context has to wait until jbd2 thread has done its work. Therefore, jbd2
thread does not need any locks to access a transaction_t (or just parts
of it?) during commit phase.
Is that correct?

If so: I was thinking whether it make sense to ignore all memory
accesses to a transaction_t (or parts of it) that happen in the commit
phase. They deliberately ignore the locking policy, and would confuse
our approach.

Is the commit phase performed by jbd2_journal_commit_transaction()?
We would add this function to our blacklist for transaction_t.

- Alex
-- 
Technische Universität Dortmund
Alexander Lochmann                PGP key: 0xBC3EF6FD
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