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

diff --git a/module/zfs/dsl_pool.c b/module/zfs/dsl_pool.c
index 704f034..771b265 100644 (file)
--- a/module/zfs/dsl_pool.c
+++ b/module/zfs/dsl_pool.c
@@ -143,7 +143,7 @@ dsl_pool_txg_history_add(dsl_pool_t *dp, uint64_t txg)
 {
        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 (file)
--- a/module/zfs/txg.c
+++ b/module/zfs/txg.c
@@ -367,6 +367,13 @@ txg_quiesce(dsl_pool_t *dp, uint64_t txg)
        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 @@ txg_quiesce(dsl_pool_t *dp, uint64_t txg)
        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();