linux-ext4 - Re: [v2] Updated locking documentation for transaction

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Date:   Mon, 8 Apr 2019 10:32:35 +0200
From:   Alexander Lochmann <alexander.lochmann@...dortmund.de>
To:     "Theodore Ts'o" <tytso@....edu>,
        Horst Schirmeier <horst.schirmeier@...dortmund.de>,
        Jan Kara <jack@...e.com>, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v2] Updated locking documentation for transaction_t

Thanks, Ted, for your feedback!
I'll submit a modified version.

- Alex

On 07.04.19 18:52, Theodore Ts'o wrote:
> On Mon, Mar 18, 2019 at 07:42:37PM +0100, Alexander Lochmann wrote:
>>  	/*t
>> -	 * Where in the log does this transaction's commit start? [no locking]
>> +	 * Where in the log does this transaction's commit start?
>> +	 * [journal_t.j_state_lock]
>>  	 */
>>  	unsigned long		t_log_start;
> 
> Well, technically, that's not quite right.  It's only assigned in one
> location, and we hold j_state_lock, yes.  But that's because we need
> to access journal->j_head.  At the point where we set t_log_start, the
> transaction has already been locked down (transaction->t_state >
> T_LOCKED).
> 
> Similarly, we happen to be holding j_state where it is currently being
> accessed, but it's not because we needed the lock in order to access
> t_log_start safely.
> 
>>  	/*
>> -	 * When transaction started
>> +	 * When transaction started [journal_t.j_state_lock]
>>  	 */
>>  	unsigned long		t_start;
> 
> And again, not really.  The primary place where t_start is set is when
> the transaction is firstt created, before it's visible anywhere else.
> after that, it is used exclusively by the commit thread, and so no
> locking is necessary.  It's true that in the places where it is used,
> j_state_lock happens to be taken, but it's strictly not necessary.
> 
>>  
>>  	/*
>> -	 * When commit was requested
>> +	 * When commit was requested [journal_t.j_state_lock]
>>  	 */
>>  	unsigned long		t_requested;
> 
> Yes, that appears to be correct.
> 
>>  
>>  	/*
>> -	 * Checkpointing stats [j_checkpoint_sem]
>> +	 * Checkpointing stats [journal_t.j_list_lock]
>>  	 */
>>  	struct transaction_chp_stats_s t_chp_stats;
>>
> 
> This appears to be correct.
> 
> 						- Ted
> 

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