linux-kernel - Re: [PATCH 6/6] jbd2: No need to use t_handle_lock in jbd2_journal_wait

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Date:   Thu, 13 Jan 2022 12:27:49 +0100
From:   Jan Kara <jack@...e.cz>
To:     Ritesh Harjani <riteshh@...ux.ibm.com>
Cc:     linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, Jan Kara <jack@...e.com>,
        Andreas Dilger <adilger.kernel@...ger.ca>, tytso@....edu,
        Eric Whitney <enwlinux@...il.com>
Subject: Re: [PATCH 6/6] jbd2: No need to use t_handle_lock in
 jbd2_journal_wait_updates

On Thu 13-01-22 08:56:29, Ritesh Harjani wrote:
> Since jbd2_journal_wait_updates() uses waitq based on t_updates atomic_t
> variable. So from code review it looks like we don't need to use
> t_handle_lock spinlock for checking t_updates value.
> Hence this patch gets rid of the spinlock protection in
> jbd2_journal_wait_updates()
> 
> Signed-off-by: Ritesh Harjani <riteshh@...ux.ibm.com>

This patch looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

Actually looking at it, t_handle_lock seems to be very much unused. I agree
we don't need it when waiting for outstanding handles but the only
remaining uses are:

1) jbd2_journal_extend() where it is not needed either - we use
atomic_add_return() to manipulate t_outstanding_credits and hold
j_state_lock for reading which provides us enough exclusion.

2) update_t_max_wait() - this is the only valid use of t_handle_lock but we
can just switch it to cmpxchg loop with a bit of care. Something like:

	unsigned long old;

	ts = jbd2_time_diff(ts, transaction->t_start);
	old = transaction->t_max_wait;
	while (old < ts)
		old = cmpxchg(&transaction->t_max_wait, old, ts);

So perhaps you can add two more patches to remove other t_handle_lock uses
and drop it completely.

								Honza

> ---
>  include/linux/jbd2.h | 4 ----
>  1 file changed, 4 deletions(-)
> 
> diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
> index 34b051aa9009..9bef47622b9d 100644
> --- a/include/linux/jbd2.h
> +++ b/include/linux/jbd2.h
> @@ -1768,22 +1768,18 @@ static inline void jbd2_journal_wait_updates(journal_t *journal)
>  	if (!commit_transaction)
>  		return;
>  
> -	spin_lock(&commit_transaction->t_handle_lock);
>  	while (atomic_read(&commit_transaction->t_updates)) {
>  		DEFINE_WAIT(wait);
>  
>  		prepare_to_wait(&journal->j_wait_updates, &wait,
>  					TASK_UNINTERRUPTIBLE);
>  		if (atomic_read(&commit_transaction->t_updates)) {
> -			spin_unlock(&commit_transaction->t_handle_lock);
>  			write_unlock(&journal->j_state_lock);
>  			schedule();
>  			write_lock(&journal->j_state_lock);
> -			spin_lock(&commit_transaction->t_handle_lock);
>  		}
>  		finish_wait(&journal->j_wait_updates, &wait);
>  	}
> -	spin_unlock(&commit_transaction->t_handle_lock);
>  }
>  
>  /*
> -- 
> 2.31.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR