Re: [PATCH] xfs: don't call into blockgc scan with freeze protection

["Darrick J. Wong" <djwong@kernel.org>]

On Thu, Feb 18, 2021 at 03:14:58PM -0500, Brian Foster wrote:
> fstest xfs/167 produced a lockdep splat that complained about a
> nested transaction acquiring freeze protection during an eofblocks
> scan. Drop freeze protection around the block reclaim scan in the
> transaction allocation code to avoid this problem.
> 
> Signed-off-by: Brian Foster <bfoster@redhat.com>

I think it seems reasonable, though I really wish that other patchset to
clean up all the "modify thread state when we start/end transactions"
had landed.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> ---
>  fs/xfs/xfs_trans.c | 19 ++++++++++++++-----
>  1 file changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/xfs/xfs_trans.c b/fs/xfs/xfs_trans.c
> index 44f72c09c203..c32c62d3b77a 100644
> --- a/fs/xfs/xfs_trans.c
> +++ b/fs/xfs/xfs_trans.c
> @@ -261,6 +261,7 @@ xfs_trans_alloc(
>  {
>  	struct xfs_trans	*tp;
>  	int			error;
> +	bool			retried = false;
>  
>  	/*
>  	 * Allocate the handle before we do our freeze accounting and setting up
> @@ -288,19 +289,27 @@ xfs_trans_alloc(
>  	INIT_LIST_HEAD(&tp->t_dfops);
>  	tp->t_firstblock = NULLFSBLOCK;
>  
> +retry:
>  	error = xfs_trans_reserve(tp, resp, blocks, rtextents);
> -	if (error == -ENOSPC) {
> +	if (error == -ENOSPC && !retried) {
>  		/*
>  		 * We weren't able to reserve enough space for the transaction.
>  		 * Flush the other speculative space allocations to free space.
>  		 * Do not perform a synchronous scan because callers can hold
>  		 * other locks.
>  		 */
> +		retried = true;
> +		if (!(flags & XFS_TRANS_NO_WRITECOUNT))
> +			sb_end_intwrite(mp->m_super);
>  		error = xfs_blockgc_free_space(mp, NULL);
> -		if (!error)
> -			error = xfs_trans_reserve(tp, resp, blocks, rtextents);
> -	}
> -	if (error) {
> +		if (error) {
> +			kmem_cache_free(xfs_trans_zone, tp);
> +			return error;
> +		}
> +		if (!(flags & XFS_TRANS_NO_WRITECOUNT))
> +			sb_start_intwrite(mp->m_super);
> +		goto retry;
> +	} else if (error) {
>  		xfs_trans_cancel(tp);
>  		return error;
>  	}
> -- 
> 2.26.2
>