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

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Date:   Thu, 11 Feb 2021 14:13:42 +0100
From:   Jan Kara <jack@...e.cz>
To:     Alexander Lochmann <alexander.lochmann@...dortmund.de>
Cc:     Jan Kara <jack@...e.cz>,
        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 Thu 11-02-21 10:53:51, Alexander Lochmann wrote:
> 
> 
> On 11.02.21 10:30, Jan Kara wrote:
> > >   	 */
> > >   	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.
> Mhm I see.
> What about '[..., no locks needed for jbd2 thread]'?

Sounds good to me.

> How do the others wait for the commit to be finished?

Well, usually they just don't touch buffers belonging to the committing
transation, they just store in b_next_transaction that after commit is
done, buffer should be added to the currently running transaction. There
are some exceptions though - e.g. jbd2_journal_invalidatepage() (called
from truncate code) which returns EBUSY in some rare cases and we use
jbd2_log_wait_commit() in ext4_wait_for_tail_page_commit() to wait for
commit to be done before we know it is safe to destroy the buffer.

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