Re: [PATCH 4/4] xfs: don't wait on future iclogs when pushing the CIL

Brian Foster
2021-06-22T12:42:04Z

On Tue, Jun 22, 2021 at 02:06:04PM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> The iclogbuf ring attached to the struct xlog is circular, hence the
> first and last iclogs in the ring can only be determined by
> comparing them against the log->l_iclog pointer.
>
> In xfs_cil_push_work(), we want to wait on previous iclogs that were
> issued so that we can flush them to stable storage with the commit
> record write, and it simply waits on the previous iclog in the ring.
> This, however, leads to CIL push hangs in generic/019 like so:
>
> task:kworker/u33:0 state:D stack:12680 pid: 7 ppid: 2 flags:0x00004000
> Workqueue: xfs-cil/pmem1 xlog_cil_push_work
> Call Trace:
> __schedule+0x30b/0x9f0
> schedule+0x68/0xe0
> xlog_wait_on_iclog+0x121/0x190
> ? wake_up_q+0xa0/0xa0
> xlog_cil_push_work+0x994/0xa10
> ? _raw_spin_lock+0x15/0x20
> ? xfs_swap_extents+0x920/0x920
> process_one_work+0x1ab/0x390
> worker_thread+0x56/0x3d0
> ? rescuer_thread+0x3c0/0x3c0
> kthread+0x14d/0x170
> ? __kthread_bind_mask+0x70/0x70
> ret_from_fork+0x1f/0x30
>
> With other threads blocking in either xlog_state_get_iclog_space()
> waiting for iclog space or xlog_grant_head_wait() waiting for log
> reservation space.
>
> The problem here is that the previous iclog on the ring might
> actually be a future iclog. That is, if log->l_iclog points at
> commit_iclog, commit_iclog is the first (oldest) iclog in the ring
> and there are no previous iclogs pending as they have all completed
> their IO and been activated again. IOWs, commit_iclog->ic_prev
> points to an iclog that will be written in the future, not one that
> has been written in the past.
>
> Hence, in this case, waiting on the ->ic_prev iclog is incorrect
> behaviour, and depending on the state of the future iclog, we can
> end up with a circular ABA wait cycle and we hang.
>
> The fix is made more complex by the fact that many iclogs states
> cannot be used to determine if the iclog is a past or future iclog.
> Hence we have to determine past iclogs by checking the LSN of the
> iclog rather than their state. A past ACTIVE iclog will have a LSN
> of zero, while a future ACTIVE iclog will have a LSN greater than
> the current iclog. We don't wait on either of these cases.
>
> Similarly, a future iclog that hasn't completed IO will have an LSN
> greater than the current iclog and so we don't wait on them. A past
> iclog that is still undergoing IO completion will have a LSN less
> than the current iclog and those are the only iclogs that we need to
> wait on.
>
> Hence we can use the iclog LSN to determine what iclogs we need to
> wait on here.
>
> Fixes: 5fd9256ce156 ("xfs: separate CIL commit record IO")
> Reported-by: Brian Foster <bfoster@redhat.com>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---

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

> fs/xfs/xfs_log_cil.c | 35 ++++++++++++++++++++++++++++-------
> 1 file changed, 28 insertions(+), 7 deletions(-)
>
> diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
> index 27bed1d9cf29..83a932878177 100644
> --- a/fs/xfs/xfs_log_cil.c
> +++ b/fs/xfs/xfs_log_cil.c
> @@ -877,7 +877,7 @@ xlog_cil_push_work(
> * Once we attach the ctx to the iclog, a shutdown can process the
> * iclog, run the callbacks and free the ctx. The only thing preventing
> * this potential UAF situation here is that we are holding the
> - * icloglock. Hence we cannot access the ctx after we have attached the
> + * icloglock. Hence we cannot access the ctx once we have attached the
> * callbacks and dropped the icloglock.
> */
> spin_lock(&log->l_icloglock);
> @@ -900,17 +900,38 @@ xlog_cil_push_work(
> spin_unlock(&cil->xc_push_lock);
>
> /*
> - * If the checkpoint spans multiple iclogs, wait for all previous
> - * iclogs to complete before we submit the commit_iclog. In this case,
> - * the commit_iclog write needs to issue a pre-flush so that the
> - * ordering is correctly preserved down to stable storage.
> + * If the checkpoint spans multiple iclogs, wait for all previous iclogs
> + * to complete before we submit the commit_iclog. We can't use state
> + * checks for this - ACTIVE can be either a past completed iclog or a
> + * future iclog being filled, while WANT_SYNC through SYNC_DONE can be a
> + * past or future iclog awaiting IO or ordered IO completion to be run.
> + * In the latter case, if it's a future iclog and we wait on it, the we
> + * will hang because it won't get processed through to ic_force_wait
> + * wakeup until this commit_iclog is written to disk. Hence we use the
> + * iclog header lsn and compare it to the commit lsn to determine if we
> + * need to wait on iclogs or not.
> *
> * NOTE: It is not safe reference the ctx after this check as we drop
> * the icloglock if we have to wait for completion of other iclogs.
> */
> if (ctx->start_lsn != commit_lsn) {
> - xlog_wait_on_iclog(commit_iclog->ic_prev);
> - spin_lock(&log->l_icloglock);
> + xfs_lsn_t plsn;
> +
> + plsn = be64_to_cpu(commit_iclog->ic_prev->ic_header.h_lsn);
> + if (plsn && XFS_LSN_CMP(plsn, commit_lsn) < 0) {
> + /*
> + * Waiting on ic_force_wait orders the completion of
> + * iclogs older than ic_prev. Hence we only need to wait
> + * on the most recent older iclog here.
> + */
> + xlog_wait_on_iclog(commit_iclog->ic_prev);
> + spin_lock(&log->l_icloglock);
> + }
> +
> + /*
> + * We need to issue a pre-flush so that the ordering for this
> + * checkpoint is correctly preserved down to stable storage.
> + */
> commit_iclog->ic_flags |= XLOG_ICL_NEED_FLUSH;
> }
>
> --
> 2.31.1
>