linux-ext4 - Re: [PATCH 15/22] jbd2: Factor out common parts of stopping and restarting a handle

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Date:   Wed, 23 Oct 2019 18:17:24 +0200
From:   Jan Kara <jack@...e.cz>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 15/22] jbd2: Factor out common parts of stopping and
 restarting a handle

On Mon 21-10-19 13:49:33, Theodore Y. Ts'o wrote:
> On Fri, Oct 04, 2019 at 12:06:01AM +0200, Jan Kara wrote:
> > jbd2__journal_restart() has quite some code that is common with
> > jbd2_journal_stop(). Factor this functionality into stop_this_handle()
> > helper and use it from both functions. Note that this also drops
> > t_handle_lock protection from jbd2__journal_restart() as
> > jbd2_journal_stop() does the same thing without it.
> > 
> > Signed-off-by: Jan Kara <jack@...e.cz>
> > ---
> >  fs/jbd2/transaction.c | 94 +++++++++++++++++++++++----------------------------
> >  1 file changed, 42 insertions(+), 52 deletions(-)
> > 
> > diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
> > index d648cec3f90f..d4ee02e5161b 100644
> > --- a/fs/jbd2/transaction.c
> > +++ b/fs/jbd2/transaction.c
> > @@ -677,52 +704,30 @@ int jbd2__journal_restart(handle_t *handle, int nblocks, gfp_t gfp_mask)
> 
> > -	read_lock(&journal->j_state_lock);
> > -	spin_lock(&transaction->t_handle_lock);
> > -	atomic_sub(handle->h_buffer_credits,
> > -		   &transaction->t_outstanding_credits);
> > -	if (handle->h_rsv_handle) {
> > -		sub_reserved_credits(journal,
> > -				     handle->h_rsv_handle->h_buffer_credits);
> > -	}
> > -	if (atomic_dec_and_test(&transaction->t_updates))
> > -		wake_up(&journal->j_wait_updates);
> > -	tid = transaction->t_tid;
> > -	spin_unlock(&transaction->t_handle_lock);
> > +	jbd_debug(2, "restarting handle %p\n", handle);
> > +	stop_this_handle(handle);
> >  	handle->h_transaction = NULL;
> > -	current->journal_info = NULL;
> >  
> > -	jbd_debug(2, "restarting handle %p\n", handle);
> > +	read_lock(&journal->j_state_lock);
> >  	need_to_start = !tid_geq(journal->j_commit_request, tid);
> >  	read_unlock(&journal->j_state_lock);
> 
> What is j_state_lock protecting at this point?  There's only a 32-bit
> read of j_commit_request at this point.

We could almost drop the lock. To be fully correct, we'd then need to use
READ_ONCE here and WRITE_ONCE in places changing j_commit_request (reasons
are well summarized in recent LWN series on how compiler can screw your
unlocked reads and writes). So probably a fair cleanup but something I've
decided to leave for later.

								Honza

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