Re: [PATCH 2/7] xfs: check buffer pin state after locking in delwri_submit

Chandan Babu R
2022-03-15T10:04:36Z

On 15 Mar 2022 at 12:12, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> AIL flushing can get stuck here:
>
> [316649.005769] INFO: task xfsaild/pmem1:324525 blocked for more than 123 seconds.
> [316649.007807] Not tainted 5.17.0-rc6-dgc+ #975
> [316649.009186] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [316649.011720] task:xfsaild/pmem1 state:D stack:14544 pid:324525 ppid: 2 flags:0x00004000
> [316649.014112] Call Trace:
> [316649.014841] <TASK>
> [316649.015492] __schedule+0x30d/0x9e0
> [316649.017745] schedule+0x55/0xd0
> [316649.018681] io_schedule+0x4b/0x80
> [316649.019683] xfs_buf_wait_unpin+0x9e/0xf0
> [316649.021850] __xfs_buf_submit+0x14a/0x230
> [316649.023033] xfs_buf_delwri_submit_buffers+0x107/0x280
> [316649.024511] xfs_buf_delwri_submit_nowait+0x10/0x20
> [316649.025931] xfsaild+0x27e/0x9d0
> [316649.028283] kthread+0xf6/0x120
> [316649.030602] ret_from_fork+0x1f/0x30
>
> in the situation where flushing gets preempted between the unpin
> check and the buffer trylock under nowait conditions:
>
> blk_start_plug(&plug);
> list_for_each_entry_safe(bp, n, buffer_list, b_list) {
> if (!wait_list) {
> if (xfs_buf_ispinned(bp)) {
> pinned++;
> continue;
> }
> Here >>>>>>
> if (!xfs_buf_trylock(bp))
> continue;
>
> This means submission is stuck until something else triggers a log
> force to unpin the buffer.
>
> To get onto the delwri list to begin with, the buffer pin state has
> already been checked, and hence it's relatively rare we get a race
> between flushing and encountering a pinned buffer in delwri
> submission to begin with. Further, to increase the pin count the
> buffer has to be locked, so the only way we can hit this race
> without failing the trylock is to be preempted between the pincount
> check seeing zero and the trylock being run.
>
> Hence to avoid this problem, just invert the order of trylock vs
> pin check. We shouldn't hit that many pinned buffers here, so
> optimising away the trylock for pinned buffers should not matter for
> performance at all.

Looks good to me from the perspective of logical correctness.

Reviewed-by: Chandan Babu R <chandan.babu@oracle.com>

>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
> fs/xfs/xfs_buf.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> index b45e0d50a405..8867f143598e 100644
> --- a/fs/xfs/xfs_buf.c
> +++ b/fs/xfs/xfs_buf.c
> @@ -2094,12 +2094,13 @@ xfs_buf_delwri_submit_buffers(
> blk_start_plug(&plug);
> list_for_each_entry_safe(bp, n, buffer_list, b_list) {
> if (!wait_list) {
> + if (!xfs_buf_trylock(bp))
> + continue;
> if (xfs_buf_ispinned(bp)) {
> + xfs_buf_unlock(bp);
> pinned++;
> continue;
> }
> - if (!xfs_buf_trylock(bp))
> - continue;
> } else {
> xfs_buf_lock(bp);
> }

--
chandan