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

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Date:   Thu, 11 Feb 2021 10:30:27 +0100
From:   Jan Kara <jack@...e.cz>
To:     Alexander Lochmann <alexander.lochmann@...dortmund.de>
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 Wed 10-02-21 10:57:39, Alexander Lochmann wrote:
> Some members of transaction_t are allowed to be read without
> any lock being held if consistency doesn't matter.
> Based on LockDoc's findings, we extended the locking
> documentation of those members.
> Each one of them is marked with a short comment:
> "no lock for quick racy checks".
> 
> Signed-off-by: Alexander Lochmann <alexander.lochmann@...dortmund.de>
> Signed-off-by: Horst Schirmeier <horst.schirmeier@...dortmund.de>

Thanks for the patch! Some comments below...

> ---
>  include/linux/jbd2.h | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> 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.

>  	/*
>  	 * Doubly-linked circular list of all buffers reserved but not yet
> -	 * modified by this transaction [j_list_lock]
> +	 * modified by this transaction [j_list_lock, no lock for quick racy checks]
>  	 */
>  	struct journal_head	*t_reserved_list;
>
>  	/*
>  	 * Doubly-linked circular list of all metadata buffers owned by this
> -	 * transaction [j_list_lock]
> +	 * transaction [j_list_lock, no lock for quick racy checks]
>  	 */
>  	struct journal_head	*t_buffers;
> 
> @@ -631,7 +631,7 @@ struct transaction_s
>  	/*
>  	 * Doubly-linked circular list of metadata buffers being shadowed by log
>  	 * IO.  The IO buffers on the iobuf list and the shadow buffers on this
> -	 * list match each other one for one at all times. [j_list_lock]
> +	 * list match each other one for one at all times. [j_list_lock, no lock for quick racy checks]
>  	 */
>  	struct journal_head	*t_shadow_list;

The above three cases are the same as t_reserved_list.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR