Re: [PATCH 03/19] xfs: prevent deadlock trying to cover an active log

Eric Sandeen
2013-10-17T15:54:34Z

On 10/14/13 5:17 PM, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> Recent analysis of a deadlocked XFS filesystem from a kernel
> crash dump indicated that the filesystem was stuck waiting for log
> space. The short story of the hang on the RHEL6 kernel is this:
>
> - the tail of the log is pinned by an inode
> - the inode has been pushed by the xfsaild
> - the inode has been flushed to it's backing buffer and is
> currently flush locked and hence waiting for backing
> buffer IO to complete and remove it from the AIL
> - the backing buffer is marked for write - it is on the
> delayed write queue
> - the inode buffer has been modified directly and logged
> recently due to unlinked inode list modification
> - the backing buffer is pinned in memory as it is in the
> active CIL context.
> - the xfsbufd won't start buffer writeback because it is
> pinned
> - xfssyncd won't force the log because it sees the log as
> needing to be covered and hence wants to issue a dummy
> transaction to move the log covering state machine along.
>
> Hence there is no trigger to force the CIL to the log and hence
> unpin the inode buffer and therefore complete the inode IO, remove
> it from the AIL and hence move the tail of the log along, allowing
> transactions to start again.
>
> Mainline kernels also have the same deadlock, though the signature
> is slightly different - the inode buffer never reaches the delayed
> write lists because xfs_buf_item_push() sees that it is pinned and
> hence never adds it to the delayed write list that the xfsaild
> flushes.
>
> There are two possible solutions here. The first is to simply force
> the log before trying to cover the log and so ensure that the CIL is
> emptied before we try to reserve space for the dummy transaction in
> the xfs_log_worker(). While this might work most of the time, it is
> still racy and is no guarantee that we don't get stuck in
> xfs_trans_reserve waiting for log space to come free. Hence it's not
> the best way to solve the problem.
>
> The second solution is to modify xfs_log_need_covered() to be aware
> of the CIL. We only should be attempting to cover the log if there
> is no current activity in the log - covering the log is the process
> of ensuring that the head and tail in the log on disk are identical
> (i.e. the log is clean and at idle). Hence, by definition, if there
> are items in the CIL then the log is not at idle and so we don't
> need to attempt to cover it.
>
> When we don't need to cover the log because it is active or idle, we
> issue a log force from xfs_log_worker() - if the log is idle, then
> this does nothing. However, if the log is active due to there being
> items in the CIL, it will force the items in the CIL to the log and
> unpin them.
>
> In the case of the above deadlock scenario, instead of
> xfs_log_worker() getting stuck in xfs_trans_reserve() attempting to
> cover the log, it will instead force the log, thereby unpinning the
> inode buffer, allowing IO to be issued and complete and hence
> removing the inode that was pinning the tail of the log from the
> AIL. At that point, everything will start moving along again. i.e.
> the xfs_log_worker turns back into a watchdog that can alleviate
> deadlocks based around pinned items that prevent the tail of the log
> from being moved...
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>

Reviewed-by: Eric Sandeen <sandeen@redhat.com>

> ---
> fs/xfs/xfs_log.c | 48 +++++++++++++++++++++++++++++-------------------
> fs/xfs/xfs_log_cil.c | 14 ++++++++++++++
> fs/xfs/xfs_log_priv.h | 10 ++++------
> 3 files changed, 47 insertions(+), 25 deletions(-)
>
> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> index a2dea108..613ed94 100644
> --- a/fs/xfs/xfs_log.c
> +++ b/fs/xfs/xfs_log.c
> @@ -1000,27 +1000,34 @@ xfs_log_space_wake(
> }
>
> /*
> - * Determine if we have a transaction that has gone to disk
> - * that needs to be covered. To begin the transition to the idle state
> - * firstly the log needs to be idle (no AIL and nothing in the iclogs).
> - * If we are then in a state where covering is needed, the caller is informed
> - * that dummy transactions are required to move the log into the idle state.
> + * Determine if we have a transaction that has gone to disk that needs to be
> + * covered. To begin the transition to the idle state firstly the log needs to
> + * be idle. That means the CIL, the AIL and the iclogs needs to be empty before
> + * we start attempting to cover the log.
> *
> - * Because this is called as part of the sync process, we should also indicate
> - * that dummy transactions should be issued in anything but the covered or
> - * idle states. This ensures that the log tail is accurately reflected in
> - * the log at the end of the sync, hence if a crash occurrs avoids replay
> - * of transactions where the metadata is already on disk.
> + * Only if we are then in a state where covering is needed, the caller is
> + * informed that dummy transactions are required to move the log into the idle
> + * state.
> + *
> + * If there are any items in the AIl or CIL, then we do not want to attempt to
> + * cover the log as we may be in a situation where there isn't log space
> + * available to run a dummy transaction and this can lead to deadlocks when the
> + * tail of the log is pinned by an item that is modified in the CIL. Hence
> + * there's no point in running a dummy transaction at this point because we
> + * can't start trying to idle the log until both the CIL and AIL are empty.
> */
> int
> xfs_log_need_covered(xfs_mount_t *mp)
> {
> - int needed = 0;
> struct xlog *log = mp->m_log;
> + int needed = 0;
>
> if (!xfs_fs_writable(mp))
> return 0;
>
> + if (!xlog_cil_empty(log))
> + return 0;
> +
> spin_lock(&log->l_icloglock);
> switch (log->l_covered_state) {
> case XLOG_STATE_COVER_DONE:
> @@ -1029,14 +1036,17 @@ xfs_log_need_covered(xfs_mount_t *mp)
> break;
> case XLOG_STATE_COVER_NEED:
> case XLOG_STATE_COVER_NEED2:
> - if (!xfs_ail_min_lsn(log->l_ailp) &&
> - xlog_iclogs_empty(log)) {
> - if (log->l_covered_state == XLOG_STATE_COVER_NEED)
> - log->l_covered_state = XLOG_STATE_COVER_DONE;
> - else
> - log->l_covered_state = XLOG_STATE_COVER_DONE2;
> - }
> - /* FALLTHRU */
> + if (xfs_ail_min_lsn(log->l_ailp))
> + break;
> + if (!xlog_iclogs_empty(log))
> + break;
> +
> + needed = 1;
> + if (log->l_covered_state == XLOG_STATE_COVER_NEED)
> + log->l_covered_state = XLOG_STATE_COVER_DONE;
> + else
> + log->l_covered_state = XLOG_STATE_COVER_DONE2;
> + break;
> default:
> needed = 1;
> break;
> diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
> index cfe9797..da8524e77 100644
> --- a/fs/xfs/xfs_log_cil.c
> +++ b/fs/xfs/xfs_log_cil.c
> @@ -711,6 +711,20 @@ xlog_cil_push_foreground(
> xlog_cil_push(log);
> }
>
> +bool
> +xlog_cil_empty(
> + struct xlog *log)
> +{
> + struct xfs_cil *cil = log->l_cilp;
> + bool empty = false;
> +
> + spin_lock(&cil->xc_push_lock);
> + if (list_empty(&cil->xc_cil))
> + empty = true;
> + spin_unlock(&cil->xc_push_lock);
> + return empty;
> +}
> +
> /*
> * Commit a transaction with the given vector to the Committed Item List.
> *
> diff --git a/fs/xfs/xfs_log_priv.h b/fs/xfs/xfs_log_priv.h
> index 136654b..f80cff2 100644
> --- a/fs/xfs/xfs_log_priv.h
> +++ b/fs/xfs/xfs_log_priv.h
> @@ -514,12 +514,10 @@ xlog_assign_grant_head(atomic64_t *head, int cycle, int space)
> /*
> * Committed Item List interfaces
> */
> -int
> -xlog_cil_init(struct xlog *log);
> -void
> -xlog_cil_init_post_recovery(struct xlog *log);
> -void
> -xlog_cil_destroy(struct xlog *log);
> +int xlog_cil_init(struct xlog *log);
> +void xlog_cil_init_post_recovery(struct xlog *log);
> +void xlog_cil_destroy(struct xlog *log);
> +bool xlog_cil_empty(struct xlog *log);
>
> /*
> * CIL force routines