linux-ext4 - [RFC] Fine-grained locking documentation for jbd2 data structures

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Date:   Thu, 15 Oct 2020 15:56:27 +0200
From:   Alexander Lochmann <alexander.lochmann@...dortmund.de>
To:     tytso@....edu, Jan Kara <jack@...e.com>
Cc:     Horst Schirmeier <horst.schirmeier@...dortmund.de>,
        linux-ext4@...r.kernel.org
Subject: [RFC] Fine-grained locking documentation for jbd2 data structures

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.
From 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

-- 
Technische Universität 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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