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Subject: [RFC] Fine-grained locking documentation for jbd2 data structures
To:     tytso@mit.edu, Jan Kara <jack@suse.com>
Cc:     Horst Schirmeier <horst.schirmeier@tu-dortmund.de>,
        linux-ext4@vger.kernel.org
References: <20190408083500.66759-1-alexander.lochmann@tu-dortmund.de>
From:   Alexander Lochmann <alexander.lochmann@tu-dortmund.de>
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Message-ID: <7827d153-f75c-89a2-1890-86e85f86c704@tu-dortmund.de>
Date:   Thu, 15 Oct 2020 15:56:27 +0200
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From: Alexander Lochmann <alexander.lochmann@tu-dortmund.de>
To: tytso@mit.edu, Jan Kara <jack@suse.com>
Cc: Horst Schirmeier <horst.schirmeier@tu-dortmund.de>,
 linux-ext4@vger.kernel.org
Message-ID: <7827d153-f75c-89a2-1890-86e85f86c704@tu-dortmund.de>
Subject: [RFC] Fine-grained locking documentation for jbd2 data structures
References: <20190408083500.66759-1-alexander.lochmann@tu-dortmund.de>
In-Reply-To: <20190408083500.66759-1-alexander.lochmann@tu-dortmund.de>

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Hi folks,

when comparing our generated locking documentation with the current
documentation
located in include/linux/jbd2.h, I found some inconsistencies. (Our
approach: https://dl.acm.org/doi/10.1145/3302424.3303948)
According to the official documentation, the following members should be
read using a lock:
journal_t
- j_flags: j_state_lock
- j_barrier_count: j_state_lock
- j_running_transaction: j_state_lock
- j_commit_sequence: j_state_lock
- j_commit_request: j_state_lock
transactiont_t
- t_nr_buffers: j_list_lock
- t_buffers: j_list_lock
- t_reserved_list: j_list_lock
- t_shadow_list: j_list_lock
jbd2_inode
- i_transaction: j_list_lock
- i_next_transaction: j_list_lock
- i_flags: j_list_lock
- i_dirty_start: j_list_lock
- i_dirty_start: j_list_lock

However, our results say that no locks are needed at all for *reading*
those members.
=46rom what I know, it is common wisdom that word-sized data types can be=

read without any lock in the Linux kernel.
All of the above members have word size, i.e., int, long, or ptr.
Is it therefore safe to split the locking documentation as follows?
@j_flags: General journaling state flags [r:nolocks, w:j_state_lock]

The following members are not word-sizes. Our results also suggest that
no locks are needed.
Can the proposed change be applied to them as well?
transaction_t
- t_chp_stats: j_checkpoint_sem
jbd2_inode:
- i_list: j_list_lock

Cheers,
Alex

--=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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