ddt prune: Add SCL_ZIO deadlock workaround #17793
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ikozhukhov
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Sep 26, 2025
behlendorf
approved these changes
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]>
ihoro
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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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Sep 29, 2025
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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.