ddt prune: Add SCL_ZIO deadlock workaround #17793
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ikozhukhov
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Sep 26, 2025
behlendorf
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Sep 26, 2025
Originally this was created for MMP, but now new cases are emerging where the same mechanism is required. Hence the name's generalization. Sponsored-by: Klara, Inc. Sponsored-by: Wasabi Technology, Inc. Signed-off-by: Igor Ostapenko <[email protected]>
Sponsored-by: Klara, Inc. Sponsored-by: Wasabi Technology, Inc. Signed-off-by: Igor Ostapenko <[email protected]>
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FYI: I've split it onto two commits:
It feels such way it must be easier to get back to this in the future. |
allanjude
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Status: Accepted
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Sponsored-by: Klara, Inc. Sponsored-by: Wasabi Technology, Inc. Reviewed-by: Brian Behlendorf <[email protected]> Reviewed-by: Allan Jude <[email protected]> Reviewed-by: Igor Kozhukhov <[email protected]> Signed-off-by: Igor Ostapenko <[email protected]> Closes #17793
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I wonder why |
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Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Motivation and Context
The situation seems to be mirror related only and it happens if one logic path takes
SCL_ZIOreader lock, then another one landsscl_write_wantedby waiting forSCL_ZIOwriter lock, and the first one asks forSCL_ZIOreader lock again.Currently, pruning code path has only two entry points:
zpool pruneCLI andztest_ddt_prune()test. Both of them callddt_prune_unique_entries(), which begins and ends the pruning process by switching thespa->spa_active_ddt_prunebool flag.The following is the actual example of the case happened with ztest. The paragraph number depicts the sequence of events.
1
ztest_ddt_prune()test running in a separate ztest thread enqueuesdsl_sync_task(prune_candidates_sync), and keeps waiting for a txg_sync_thread to get it done.2 The
txg_sync_thread()runsdsl_pool_sync()which invokesprune_candidates_syncsync task. Theprune_candidates_sync()takesSCL_ZIOreader lock before the actual work.3 Another thread asks and waits for
SCL_ZIOwriter lock viaspa_vdev_state_enter(), in this case it wasztest_scrub. The lock getsscl_write_wanted++.4 The
txg_sync_thread()continues runningprune_candidates_sync(), eventually it hitszio_vdev_io_start()which decides to takeSCL_ZIOreader lock. And havingscl_write_wanted > 0it is not going to happen, as theztest_scrubthread actually waits for the pruning process to finish and freeSCL_ZIOreader lock.Description
The
mmp_flagofspa_config_enter_impl()is used to ignore pending write locks. And the condition for this isspa->spa_spa_active_ddt_pruneflag.As long as such change makes
spa_config_enter_mmp()function have general application it is proposed to rename it tospa_config_enter_priority().How Has This Been Tested?
Types of changes
Checklist:
Signed-off-by.