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dn_struct_rwlock can not be held in dmu_tx_try_assign() #8929

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Jun 22, 2019
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dn_struct_rwlock can not be held in dmu_tx_try_assign() #8929

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19 changes: 19 additions & 0 deletions module/zfs/dmu_tx.c
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Original file line number Diff line number Diff line change
Expand Up @@ -925,6 +925,25 @@ dmu_tx_try_assign(dmu_tx_t *tx, uint64_t txg_how)
txh = list_next(&tx->tx_holds, txh)) {
dnode_t *dn = txh->txh_dnode;
if (dn != NULL) {
/*
* This thread can't hold the dn_struct_rwlock
* while assigning the tx, because this can lead to
* deadlock. Specifically, if this dnode is already
* assigned to an earlier txg, this thread may need
* to wait for that txg to sync (the ERESTART case
* below). The other thread that has assigned this
* dnode to an earlier txg prevents this txg from
* syncing until its tx can complete (calling
* dmu_tx_commit()), but it may need to acquire the
* dn_struct_rwlock to do so (e.g. via
* dmu_buf_hold*()).
*
* Note that this thread can't hold the lock for
* read either, but the rwlock doesn't record
* enough information to make that assertion.
*/
ASSERT(!RW_WRITE_HELD(&dn->dn_struct_rwlock));

mutex_enter(&dn->dn_mtx);
if (dn->dn_assigned_txg == tx->tx_txg - 1) {
mutex_exit(&dn->dn_mtx);
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