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OpenZFS 9187 - racing condition between vdev label and spa_last_synced_txg in vdev_validate 

ztest failed with uncorrectable IO error despite having the fix for
7163.  Both sides of the mirror have CANT_OPEN_BAD_LABEL, which also
distinguishes it from that issue.

Definitely seems like a racing condition between the vdev_validate
and spa_sync:
1. Thread A (spa_sync): vdev label is updated to latest txg
2. Thread B (vdev_validate): vdev label's txg is compared to
   spa_last_synced_txg and is ahead.
3. Thread A (spa_sync): spa_last_synced_txg is updated to latest txg.

Solution: do not check txg in vdev_validate unless config lock is held.

Authored by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Matt Ahrens <matthew.ahrens@delphix.com>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Approved by: Robert Mustacchi <rm@joyent.com>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://illumos.org/issues/9187
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/805fda72
Closes #7529
This commit is contained in:
Pavel Zakharov 2016-10-19 17:46:08 -04:00 committed by Brian Behlendorf
parent b669ab83bb
commit a11c7aaec9
1 changed files with 4 additions and 1 deletions
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5
module/zfs/vdev.c
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@ -1692,8 +1692,11 @@ vdev_validate(vdev_t *vd)
/*
* If we are performing an extreme rewind, we allow for a label that
* was modified at a point after the current txg.
* If config lock is not held do not check for the txg. spa_sync could
* be updating the vdev's label before updating spa_last_synced_txg.
*/
if (spa->spa_extreme_rewind || spa_last_synced_txg(spa) == 0)
if (spa->spa_extreme_rewind || spa_last_synced_txg(spa) == 0 ||
spa_config_held(spa, SCL_CONFIG, RW_WRITER) != SCL_CONFIG)
txg = UINT64_MAX;
else
txg = spa_last_synced_txg(spa);