Btrfs: fix deadlock with nested trans handles
Zach found this deadlock that would happen like this btrfs_end_transaction <- reduce trans->use_count to 0 btrfs_run_delayed_refs btrfs_cow_block find_free_extent btrfs_start_transaction <- increase trans->use_count to 1 allocate chunk btrfs_end_transaction <- decrease trans->use_count to 0 btrfs_run_delayed_refs lock tree block we are cowing above ^^ We need to only decrease trans->use_count if it is above 1, otherwise leave it alone. This will make nested trans be the only ones who decrease their added ref, and will let us get rid of the trans->use_count++ hack if we have to commit the transaction. Thanks, cc: stable@vger.kernel.org Reported-by: Zach Brown <zab@redhat.com> Signed-off-by: Josef Bacik <jbacik@fb.com> Tested-by: Zach Brown <zab@redhat.com> Signed-off-by: Chris Mason <clm@fb.com>
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@ -683,7 +683,8 @@ static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
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int lock = (trans->type != TRANS_JOIN_NOLOCK);
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int err = 0;
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if (--trans->use_count) {
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if (trans->use_count > 1) {
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trans->use_count--;
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trans->block_rsv = trans->orig_rsv;
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return 0;
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}
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@ -731,18 +732,11 @@ static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
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}
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if (lock && ACCESS_ONCE(cur_trans->state) == TRANS_STATE_BLOCKED) {
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if (throttle) {
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/*
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* We may race with somebody else here so end up having
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* to call end_transaction on ourselves again, so inc
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* our use_count.
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*/
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trans->use_count++;
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if (throttle)
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return btrfs_commit_transaction(trans, root);
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} else {
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else
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wake_up_process(info->transaction_kthread);
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}
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}
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if (trans->type & __TRANS_FREEZABLE)
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sb_end_intwrite(root->fs_info->sb);
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