Re: [PATCH 2/4] xfs: remove callback dequeue loop from xlog_state_do_iclog_callbacks

[Brian Foster <bfoster@redhat.com>]

On Tue, Jun 22, 2021 at 02:06:02PM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> If we are processing callbacks on an iclog, nothing can be
> concurrently adding callbacks to the loop. We only add callbacks to
> the iclog when they are in ACTIVE or WANT_SYNC state, and we
> explicitly do not add callbacks if the iclog is already in IOERROR
> state.
> 
> The only way to have a dequeue racing with an enqueue is to be
> processing a shutdown without a direct reference to an iclog in
> ACTIVE or WANT_SYNC state. As the enqueue avoids this race
> condition, we only ever need a single dequeue operation in
> xlog_state_do_iclog_callbacks(). Hence we can remove the loop.
> 

This sort of relates to my question on the previous patch..

> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---

Reviewed-by: Brian Foster <bfoster@redhat.com>

>  fs/xfs/xfs_log.c | 16 ++++++----------
>  1 file changed, 6 insertions(+), 10 deletions(-)
> 
> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> index bb4390942275..05b00fa4d661 100644
> --- a/fs/xfs/xfs_log.c
> +++ b/fs/xfs/xfs_log.c
> @@ -2774,19 +2774,15 @@ xlog_state_do_iclog_callbacks(
>  	struct xlog		*log,
>  	struct xlog_in_core	*iclog)
>  {
> -	trace_xlog_iclog_callbacks_start(iclog, _RET_IP_);
> -	spin_lock(&iclog->ic_callback_lock);
> -	while (!list_empty(&iclog->ic_callbacks)) {
> -		LIST_HEAD(tmp);
> +	LIST_HEAD(tmp);
>  
> -		list_splice_init(&iclog->ic_callbacks, &tmp);
> -
> -		spin_unlock(&iclog->ic_callback_lock);
> -		xlog_cil_process_committed(&tmp);
> -		spin_lock(&iclog->ic_callback_lock);
> -	}
> +	trace_xlog_iclog_callbacks_start(iclog, _RET_IP_);
>  
> +	spin_lock(&iclog->ic_callback_lock);
> +	list_splice_init(&iclog->ic_callbacks, &tmp);
>  	spin_unlock(&iclog->ic_callback_lock);
> +
> +	xlog_cil_process_committed(&tmp);
>  	trace_xlog_iclog_callbacks_done(iclog, _RET_IP_);
>  }
>  
> -- 
> 2.31.1
>