On Fri, Apr 22, 2022 at 10:12:26AM -0400, Brian Foster wrote:
> The filestream AG selection loop uses pagf data to aid in AG
> selection, which depends on pagf initialization. If the in-core
> structure is not initialized, the caller invokes the AGF read path
> to do so and carries on. If another task enters the loop and finds
> a pagf init already in progress, the AGF read returns -EAGAIN and
> the task continues the loop. This does not increment the current ag
> index, however, which means the task spins on the current AGF buffer
> until unlocked.
>
> If the AGF read I/O submitted by the initial task happens to be
> delayed for whatever reason, this results in soft lockup warnings
Is there a specific 'whatever reason' going on here?
> via the spinning task. This is reproduced by xfs/170. To avoid this
> problem, fix the AGF trylock failure path to properly iterate to the
> next AG. If a task iterates all AGs without making progress, the
> trylock behavior is dropped in favor of blocking locks and thus a
> soft lockup is no longer possible.
>
> Fixes: f48e2df8a877ca1c ("xfs: make xfs_*read_agf return EAGAIN to ALLOC_FLAG_TRYLOCK callers")
Ooops, this was a major braino on my part.
Reviewed-by: Darrick J. Wong <djwong@xxxxxxxxxx>
--D
> Signed-off-by: Brian Foster <bfoster@xxxxxxxxxx>
> ---
>
> I included the Fixes: tag because this looks like a regression in said
> commit, but I've not explicitly verified.
>
> Brian
>
> fs/xfs/xfs_filestream.c | 7 ++++---
> 1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/fs/xfs/xfs_filestream.c b/fs/xfs/xfs_filestream.c
> index 6a3ce0f6dc9e..be9bcf8a1f99 100644
> --- a/fs/xfs/xfs_filestream.c
> +++ b/fs/xfs/xfs_filestream.c
> @@ -128,11 +128,12 @@ xfs_filestream_pick_ag(
> if (!pag->pagf_init) {
> err = xfs_alloc_pagf_init(mp, NULL, ag, trylock);
> if (err) {
> - xfs_perag_put(pag);
> - if (err != -EAGAIN)
> + if (err != -EAGAIN) {
> + xfs_perag_put(pag);
> return err;
> + }
> /* Couldn't lock the AGF, skip this AG. */
> - continue;
> + goto next_ag;
> }
> }
>
> --
> 2.34.1
>
