To:     "Theodore Ts'o" <tytso@mit.edu>,
        Horst Schirmeier <horst.schirmeier@tu-dortmund.de>,
        Jan Kara <jack@suse.com>, linux-ext4@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20190318184237.20677-1-alexander.lochmann@tu-dortmund.de>
 <20190407165206.GA11370@mit.edu>
From:   Alexander Lochmann <alexander.lochmann@tu-dortmund.de>
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Subject: Re: [v2] Updated locking documentation for transaction_t
Message-ID: <ec3b972e-8342-8359-448d-3efaa476d5b3@tu-dortmund.de>
Date:   Mon, 8 Apr 2019 10:32:35 +0200
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From: Alexander Lochmann <alexander.lochmann@tu-dortmund.de>
To: Theodore Ts'o <tytso@mit.edu>,
 Horst Schirmeier <horst.schirmeier@tu-dortmund.de>, Jan Kara
 <jack@suse.com>, linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org
Message-ID: <ec3b972e-8342-8359-448d-3efaa476d5b3@tu-dortmund.de>
Subject: Re: [v2] Updated locking documentation for transaction_t
References: <20190318184237.20677-1-alexander.lochmann@tu-dortmund.de>
 <20190407165206.GA11370@mit.edu>
In-Reply-To: <20190407165206.GA11370@mit.edu>

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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 lockin=
g]
>> +	 * Where in the log does this transaction's commit start?
>> +	 * [journal_t.j_state_lock]
>>  	 */
>>  	unsigned long		t_log_start;
>=20
> 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).
>=20
> 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.
>=20
>>  	/*
>> -	 * When transaction started
>> +	 * When transaction started [journal_t.j_state_lock]
>>  	 */
>>  	unsigned long		t_start;
>=20
> 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.
>=20
>> =20
>>  	/*
>> -	 * When commit was requested
>> +	 * When commit was requested [journal_t.j_state_lock]
>>  	 */
>>  	unsigned long		t_requested;
>=20
> Yes, that appears to be correct.
>=20
>> =20
>>  	/*
>> -	 * Checkpointing stats [j_checkpoint_sem]
>> +	 * Checkpointing stats [journal_t.j_list_lock]
>>  	 */
>>  	struct transaction_chp_stats_s t_chp_stats;
>>
>=20
> This appears to be correct.
>=20
> 						- Ted
>=20

--=20
Technische Universit=C3=A4t Dortmund
Alexander Lochmann                PGP key: 0xBC3EF6FD
Otto-Hahn-Str. 16                 phone:  +49.231.7556141
D-44227 Dortmund                  fax:    +49.231.7556116
http://ess.cs.tu-dortmund.de/Staff/al


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