Re: [PATCH 10/11] xfs: make xfs_*read_agf return EAGAIN to ALLOC_FLAG_TRYLOCK callers

[Christoph Hellwig <hch@xxxxxxxxxxxxx>]

On Thu, Jan 16, 2020 at 10:24:41PM -0800, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> 
> Refactor xfs_read_agf and xfs_alloc_read_agf to return EAGAIN if the
> caller passed TRYLOCK and we weren't able to get the lock; and change
> the callers to recognize this.
> 
> Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> ---
>  fs/xfs/libxfs/xfs_alloc.c |   31 +++++++++++++++----------------
>  fs/xfs/libxfs/xfs_bmap.c  |    9 +++++----
>  fs/xfs/xfs_filestream.c   |   11 ++++++-----
>  3 files changed, 26 insertions(+), 25 deletions(-)
> 
> 
> diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
> index 83273975df77..26f3e4db84e0 100644
> --- a/fs/xfs/libxfs/xfs_alloc.c
> +++ b/fs/xfs/libxfs/xfs_alloc.c
> @@ -2502,13 +2502,15 @@ xfs_alloc_fix_freelist(
>  
>  	if (!pag->pagf_init) {
>  		error = xfs_alloc_read_agf(mp, tp, args->agno, flags, &agbp);
> -		if (error)
> -			goto out_no_agbp;
> -		if (!pag->pagf_init) {
> +		if (error == -EAGAIN) {
> +			/* Couldn't lock the AGF so skip this AG. */
>  			ASSERT(flags & XFS_ALLOC_FLAG_TRYLOCK);
>  			ASSERT(!(flags & XFS_ALLOC_FLAG_FREEING));
> -			goto out_agbp_relse;
> +			error = 0;
> +			goto out_no_agbp;
>  		}
> +		if (error)
> +			goto out_no_agbp;

I wonder if something like:

		if (error) {
			if (error == -EAGAIN) {
				/* Couldn't lock the AGF so skip this AG. */
	 			ASSERT(flags & XFS_ALLOC_FLAG_TRYLOCK);
	 			ASSERT(!(flags & XFS_ALLOC_FLAG_FREEING));
				error = 0;
			}
			goto out_no_agbp;
		}

would be a little nicer here?

> @@ -2533,13 +2535,15 @@ xfs_alloc_fix_freelist(
>  	 */
>  	if (!agbp) {
>  		error = xfs_alloc_read_agf(mp, tp, args->agno, flags, &agbp);
> -		if (error)
> -			goto out_no_agbp;
> -		if (!agbp) {
> +		if (error == -EAGAIN) {
> +			/* Couldn't lock the AGF so skip this AG. */
>  			ASSERT(flags & XFS_ALLOC_FLAG_TRYLOCK);
>  			ASSERT(!(flags & XFS_ALLOC_FLAG_FREEING));
> +			error = 0;
>  			goto out_no_agbp;
>  		}
> +		if (error)
> +			goto out_no_agbp;
>  	}

Same here.  Also shouldn't those asserts just move into
xfs_alloc_read_agf or go away now that we have a proper return value
and not the magic NULL buffer?

> +	error = xfs_alloc_read_agf(mp, tp, agno, flags, &bp);
> +	if (error)
>  		return error;
> +	xfs_trans_brelse(tp, bp);
>  	return 0;

Maybe simplify this further to:

	error = xfs_alloc_read_agf(mp, tp, agno, flags, &bp);
	if (!error)
		xfs_trans_brelse(tp, bp);
	return error;

> @@ -2958,12 +2962,9 @@ xfs_read_agf(
>  	trace_xfs_read_agf(mp, agno);
>  
>  	ASSERT(agno != NULLAGNUMBER);
> -	error = xfs_trans_read_buf(
> -			mp, tp, mp->m_ddev_targp,
> +	error = xfs_trans_read_buf(mp, tp, mp->m_ddev_targp,

This hunk should probably go into the patch that changed the
xfs_trans_read_buf return value instead.

> +		if (error == -EAGAIN) {
> +			/* Couldn't lock the AGF, so skip this AG. */
>  			*notinit = 1;
> +			error = 0;
>  			goto out;
>  		}
> +		if (error)
> +			goto out;

Should probably be:

		if (error) {
			if (error == -EAGAIN) {
				/* Couldn't lock the AGF, so skip this AG. */
	 			*notinit = 1;
				error = 0;
			}
 			goto out;