txg visibility code should not execute under tc_open_lock

The memory allocation and locking in `spa_txg_history_*()` can
potentially block txg_hold_open for arbitrarily long periods of time.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #4333

diff --git a/module/zfs/txg.c b/module/zfs/txg.c
index ed8007e052385dd944a147cf656de076247ceb36..9dda58c283df40b7d35df97777ad9e17f51f5430 100644 (file)
--- a/module/zfs/txg.c
+++ b/module/zfs/txg.c
@@ -365,6 +365,7 @@ static void
 txg_quiesce(dsl_pool_t *dp, uint64_t txg)
 {
        tx_state_t *tx = &dp->dp_tx;
+       uint64_t tx_open_time;
        int g = txg & TXG_MASK;
        int c;
 
@@ -376,10 +377,7 @@ txg_quiesce(dsl_pool_t *dp, uint64_t txg)
 
        ASSERT(txg == tx->tx_open_txg);
        tx->tx_open_txg++;
-       tx->tx_open_time = gethrtime();
-
-       spa_txg_history_set(dp->dp_spa, txg, TXG_STATE_OPEN, tx->tx_open_time);
-       spa_txg_history_add(dp->dp_spa, tx->tx_open_txg, tx->tx_open_time);
+       tx->tx_open_time = tx_open_time = gethrtime();
 
        DTRACE_PROBE2(txg__quiescing, dsl_pool_t *, dp, uint64_t, txg);
        DTRACE_PROBE2(txg__opened, dsl_pool_t *, dp, uint64_t, tx->tx_open_txg);
@@ -391,6 +389,9 @@ txg_quiesce(dsl_pool_t *dp, uint64_t txg)
        for (c = 0; c < max_ncpus; c++)
                mutex_exit(&tx->tx_cpu[c].tc_open_lock);
 
+       spa_txg_history_set(dp->dp_spa, txg, TXG_STATE_OPEN, tx_open_time);
+       spa_txg_history_add(dp->dp_spa, txg + 1, tx_open_time);
+
        /*
         * Quiesce the transaction group by waiting for everyone to txg_exit().
         */