From: Dave Chinner <dchinner@xxxxxxxxxx>
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.
Fix this by only waiting on the previous iclog when the commit_iclog
is not the oldest iclog in the ring.
Fixes: 5fd9256ce156 ("xfs: separate CIL commit record IO")
Reported-by: Brian Foster <bfoster@xxxxxxxxxx>
Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
---
fs/xfs/xfs_log_cil.c | 18 ++++++++++++------
1 file changed, 12 insertions(+), 6 deletions(-)
diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
index 705619e9dab4..398f00cf9cbf 100644
--- a/fs/xfs/xfs_log_cil.c
+++ b/fs/xfs/xfs_log_cil.c
@@ -1075,15 +1075,21 @@ xlog_cil_push_work(
ticket = ctx->ticket;
/*
- * 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. If the commit iclog is
+ * at the head of the iclog ring, then all other iclogs have completed
+ * and are waiting on this one and hence we don't need to wait.
+ *
+ * Regardless of whether we need to wait or not, the the commit_iclog
+ * write needs to issue a pre-flush so that the ordering for this
+ * checkpoint is correctly preserved down to stable storage.
*/
spin_lock(&log->l_icloglock);
if (ctx->start_lsn != commit_lsn) {
- xlog_wait_on_iclog(commit_iclog->ic_prev);
- spin_lock(&log->l_icloglock);
+ if (commit_iclog != log->l_iclog) {
+ xlog_wait_on_iclog(commit_iclog->ic_prev);
+ spin_lock(&log->l_icloglock);
+ }
commit_iclog->ic_flags |= XLOG_ICL_NEED_FLUSH;
}
--
2.31.1
