[PATCH 8/8] xfs: order CIL checkpoint start records

Dave Chinner
2021-06-17T08:26:25Z

From: Dave Chinner <dchinner@redhat.com>

Because log recovery depends on strictly ordered start records as
well as strictly ordered commit records.

This is a zero day bug in the way XFS writes pipelined transactions
to the journal which is exposed by commit facd77e4e38b ("xfs: CIL
work is serialised, not pipelined") which re-introduces explicit
concurrent commits back into the on-disk journal.

The XFS journal commit code has never ordered start records and we
have relied on strict commit record ordering for correct recovery
ordering of concurrently written transactions. Unfortunately, root
cause analysis uncovered the fact that log recovery uses the LSN of
the start record for transaction commit processing. Hence the
commits are processed in strict orderi by recovery, but the LSNs
associated with the commits can be out of order and so recovery may
stamp incorrect LSNs into objects and/or misorder intents in the AIL
for later processing. This can result in log recovery failures
and/or on disk corruption, sometimes silent.

Because this is a long standing log recovery issue, we can't just
fix log recovery and call it good. This still leaves older kernels
susceptible to recovery failures and corruption when replaying a log
from a kernel that pipelines checkpoints. There is also the issue
that in-memory ordering for AIL pushing and data integrity
operations are based on checkpoint start LSNs, and if the start LSN
is incorrect in the journal, it is also incorrect in memory.

Hence there's really only one choice for fixing this zero-day bug:
we need to strictly order checkpoint start records in ascending
sequence order in the log, the same way we already strictly order
commit records.

Fixes: facd77e4e38b ("xfs: CIL work is serialised, not pipelined")
Signed-off-by: Dave Chinner <dchinner@redhat.com>
---
fs/xfs/xfs_log.c | 1 +
fs/xfs/xfs_log_cil.c | 101 +++++++++++++++++++++++++++++-------------
fs/xfs/xfs_log_priv.h | 1 +
3 files changed, 71 insertions(+), 32 deletions(-)

diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
index 359246d54db7..94b6bccb9de9 100644
--- a/fs/xfs/xfs_log.c
+++ b/fs/xfs/xfs_log.c
@@ -3743,6 +3743,7 @@ xfs_log_force_umount(
* avoid races.
*/
spin_lock(&log->l_cilp->xc_push_lock);
+ wake_up_all(&log->l_cilp->xc_start_wait);
wake_up_all(&log->l_cilp->xc_commit_wait);
spin_unlock(&log->l_cilp->xc_push_lock);
xlog_state_do_callback(log);
diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
index 87e30917ce2e..722c21f21b81 100644
--- a/fs/xfs/xfs_log_cil.c
+++ b/fs/xfs/xfs_log_cil.c
@@ -684,6 +684,7 @@ xlog_cil_committed(
*/
if (abort) {
spin_lock(&ctx->cil->xc_push_lock);
+ wake_up_all(&ctx->cil->xc_start_wait);
wake_up_all(&ctx->cil->xc_commit_wait);
spin_unlock(&ctx->cil->xc_push_lock);
}
@@ -788,6 +789,10 @@ xlog_cil_build_trans_hdr(
* If the context doesn't have a start_lsn recorded, then this iclog will
* contain the start record for the checkpoint. Otherwise this write contains
* the commit record for the checkpoint.
+ *
+ * Once we've set the LSN for the given operation, wake up any ordered write
+ * waiters that can make progress now that we have a stable LSN for write
+ * ordering purposes.
*/
void
xlog_cil_set_ctx_write_state(
@@ -798,9 +803,16 @@ xlog_cil_set_ctx_write_state(
xfs_lsn_t lsn = be64_to_cpu(iclog->ic_header.h_lsn);

ASSERT(!ctx->commit_lsn);
- spin_lock(&cil->xc_push_lock);
if (!ctx->start_lsn) {
+ spin_lock(&cil->xc_push_lock);
+ /*
+ * The LSN we need to pass to the log items on transaction
+ * commit is the LSN reported by the first log vector write, not
+ * the commit lsn. If we use the commit record lsn then we can
+ * move the tail beyond the grant write head.
+ */
ctx->start_lsn = lsn;
+ wake_up_all(&cil->xc_start_wait);
spin_unlock(&cil->xc_push_lock);
return;
}
@@ -811,9 +823,6 @@ xlog_cil_set_ctx_write_state(
* context controls when the iclog is released for IO.
*/
atomic_inc(&iclog->ic_refcnt);
- ctx->commit_iclog = iclog;
- ctx->commit_lsn = lsn;
- spin_unlock(&cil->xc_push_lock);

/*
* xlog_state_get_iclog_space() guarantees there is enough space in the
@@ -827,6 +836,12 @@ xlog_cil_set_ctx_write_state(
}
list_add_tail(&ctx->iclog_entry, &iclog->ic_callbacks);
spin_unlock(&iclog->ic_callback_lock);
+
+ spin_lock(&cil->xc_push_lock);
+ ctx->commit_iclog = iclog;
+ ctx->commit_lsn = lsn;
+ wake_up_all(&cil->xc_commit_wait);
+ spin_unlock(&cil->xc_push_lock);
}

/*
@@ -834,10 +849,16 @@ xlog_cil_set_ctx_write_state(
* relies on the context LSN being zero until the log write has guaranteed the
* LSN that the log write will start at via xlog_state_get_iclog_space().
*/
+enum {
+ _START_RECORD,
+ _COMMIT_RECORD,
+};
+
static int
xlog_cil_order_write(
struct xfs_cil *cil,
- xfs_csn_t sequence)
+ xfs_csn_t sequence,
+ int record)
{
struct xfs_cil_ctx *ctx;

@@ -860,19 +881,50 @@ xlog_cil_order_write(
*/
if (ctx->sequence >= sequence)
continue;
- if (!ctx->commit_lsn) {
- /*
- * It is still being pushed! Wait for the push to
- * complete, then start again from the beginning.
- */
- xlog_wait(&cil->xc_commit_wait, &cil->xc_push_lock);
- goto restart;
+
+ /* Wait until the LSN for the record has been recorded. */
+ switch (record) {
+ case _START_RECORD:
+ if (!ctx->start_lsn) {
+ xlog_wait(&cil->xc_start_wait, &cil->xc_push_lock);
+ goto restart;
+ }
+ break;
+ case _COMMIT_RECORD:
+ if (!ctx->commit_lsn) {
+ xlog_wait(&cil->xc_commit_wait, &cil->xc_push_lock);
+ goto restart;
+ }
+ break;
+ default:
+ ASSERT(0);
+ break;
}
}
spin_unlock(&cil->xc_push_lock);
return 0;
}

+/*
+ * Write out the log vector change now attached to the CIL context. This will
+ * write a start record that needs to be strictly ordered in ascending CIL
+ * sequence order so that log recovery will always use in-order start LSNs when
+ * replaying checkpoints.
+ */
+static int
+xlog_cil_write_chain(
+ struct xfs_cil_ctx *ctx,
+ uint32_t num_bytes)
+{
+ struct xlog *log = ctx->cil->xc_log;
+ int error;
+
+ error = xlog_cil_order_write(ctx->cil, ctx->sequence, _START_RECORD);
+ if (error)
+ return error;
+ return xlog_write(log, ctx, &ctx->lv_chain, ctx->ticket, num_bytes);
+}
+
/*
* Write out the commit record of a checkpoint transaction to close off a
* running log write. These commit records are strictly ordered in ascending CIL
@@ -906,7 +958,7 @@ xlog_cil_write_commit_record(
if (XLOG_FORCED_SHUTDOWN(log))
return -EIO;

- error = xlog_cil_order_write(ctx->cil, ctx->sequence);
+ error = xlog_cil_order_write(ctx->cil, ctx->sequence, _COMMIT_RECORD);
if (error)
return error;

@@ -1125,17 +1177,10 @@ xlog_cil_push_work(
wait_for_completion(&bdev_flush);

/*
- * The LSN we need to pass to the log items on transaction commit is the
- * LSN reported by the first log vector write, not the commit lsn. If we
- * use the commit record lsn then we can move the tail beyond the grant
- * write head.
- */
- error = xlog_write(log, ctx, &ctx->lv_chain, ctx->ticket, num_bytes);
-
- /*
- * Take the lvhdr back off the lv_chain as it should not be passed
- * to log IO completion.
+ * Once we write the log vector chain, take the lvhdr back off it as it
+ * must not be passed to log IO completion.
*/
+ error = xlog_cil_write_chain(ctx, num_bytes);
list_del(&lvhdr.lv_list);
if (error)
goto out_abort_free_ticket;
@@ -1144,15 +1189,6 @@ xlog_cil_push_work(
if (error)
goto out_abort_free_ticket;

- /*
- * now the checkpoint commit is complete and we've attached the
- * callbacks to the iclog we can assign the commit LSN to the context
- * and wake up anyone who is waiting for the commit to complete.
- */
- spin_lock(&cil->xc_push_lock);
- wake_up_all(&cil->xc_commit_wait);
- spin_unlock(&cil->xc_push_lock);
-
/*
* Pull the ticket off the ctx so we can ungrant it after releasing the
* commit_iclog. The ctx may be freed by the time we return from
@@ -1728,6 +1764,7 @@ xlog_cil_init(
init_waitqueue_head(&cil->xc_push_wait);
init_rwsem(&cil->xc_ctx_lock);
init_waitqueue_head(&cil->xc_commit_wait);
+ init_waitqueue_head(&cil->xc_start_wait);
log->l_cilp = cil;

ctx = xlog_cil_ctx_alloc();
diff --git a/fs/xfs/xfs_log_priv.h b/fs/xfs/xfs_log_priv.h
index 72dfa3b89513..b807a179b916 100644
--- a/fs/xfs/xfs_log_priv.h
+++ b/fs/xfs/xfs_log_priv.h
@@ -279,6 +279,7 @@ struct xfs_cil {
bool xc_push_commit_stable;
struct list_head xc_committing;
wait_queue_head_t xc_commit_wait;
+ wait_queue_head_t xc_start_wait;
xfs_csn_t xc_current_sequence;
wait_queue_head_t xc_push_wait; /* background push throttle */

--
2.31.1