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From 57f5a2008e2e6acf58934cf43c5fdca0faffa73e Mon Sep 17 00:00:00 2001
From: Brian Behlendorf <behlendorf1@llnl.gov>
Date: Thu, 25 Apr 2013 16:29:22 -0700
Subject: [PATCH] Fix txg_quiesce thread deadlock
A deadlock was accidentally introduced by commit e95853a which
can occur when the system is under memory pressure. What happens
is that while the txg_quiesce thread is holding the tx->tx_cpu
locks it enters memory reclaim. In the context of this memory
reclaim it then issues synchronous I/O to a ZVOL swap device.
Because the txg_quiesce thread is holding the tx->tx_cpu locks
a new txg cannot be opened to handle the I/O. Deadlock.
The fix is straight forward. Move the memory allocation outside
the critical region where the tx->tx_cpu locks are held. And for
good measure change the offending allocation to KM_PUSHPAGE to
ensure it never attempts to issue I/O during reclaim.
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #1274
---
module/zfs/dsl_pool.c | 2 +-
module/zfs/txg.c | 14 +++++++-------
2 files changed, 8 insertions(+), 8 deletions(-)
diff --git a/module/zfs/dsl_pool.c b/module/zfs/dsl_pool.c
index 704f034..771b265 100644
--- a/module/zfs/dsl_pool.c
+++ b/module/zfs/dsl_pool.c
@@ -143,7 +143,7 @@
{
txg_history_t *th, *rm;
- th = kmem_zalloc(sizeof(txg_history_t), KM_SLEEP);
+ th = kmem_zalloc(sizeof(txg_history_t), KM_PUSHPAGE);
mutex_init(&th->th_lock, NULL, MUTEX_DEFAULT, NULL);
th->th_kstat.txg = txg;
th->th_kstat.state = TXG_STATE_OPEN;
diff --git a/module/zfs/txg.c b/module/zfs/txg.c
index c7c3df3..7c820af 100644
--- a/module/zfs/txg.c
+++ b/module/zfs/txg.c
@@ -367,6 +367,13 @@
tx->tx_open_txg++;
/*
+ * Now that we've incremented tx_open_txg, we can let threads
+ * enter the next transaction group.
+ */
+ for (c = 0; c < max_ncpus; c++)
+ mutex_exit(&tx->tx_cpu[c].tc_lock);
+
+ /*
* Measure how long the txg was open and replace the kstat.
*/
th = dsl_pool_txg_history_get(dp, txg);
@@ -376,13 +383,6 @@
dsl_pool_txg_history_add(dp, tx->tx_open_txg);
/*
- * Now that we've incremented tx_open_txg, we can let threads
- * enter the next transaction group.
- */
- for (c = 0; c < max_ncpus; c++)
- mutex_exit(&tx->tx_cpu[c].tc_lock);
-
- /*
* Quiesce the transaction group by waiting for everyone to txg_exit().
*/
start = gethrtime();
--
1.8.1.6
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