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Song Liu
9164e4a5af
From Yu Kuai, written by Song Liu Recent tests with raid10 revealed many issues with the following scenarios: - add or remove disks to the array - issue io to the array At first, we fixed each problem independently respect that io can concurrent with array reconfiguration. However, with more issues reported continuously, I am hoping to fix these problems thoroughly. Refer to how block layer protect io with queue reconfiguration (for example, change elevator): blk_mq_freeze_queue -> wait for all io to be done, and prevent new io to be dispatched // reconfiguration blk_mq_unfreeze_queue I think we can do something similar to synchronize io with array reconfiguration. Current synchronization works as the following. For the reconfiguration operation: 1. Hold 'reconfig_mutex'; 2. Check that rdev can be added/removed, one condition is that there is no IO (for example, check nr_pending). 3. Do the actual operations to add/remove a rdev, one procedure is set/clear a pointer to rdev. 4. Check if there is still no IO on this rdev, if not, revert the change. IO path uses rcu_read_lock/unlock() to access rdev. - rcu is used wrongly; - There are lots of places involved that old rdev can be read, however, many places doesn't handle old value correctly; - Between step 3 and 4, if new io is dispatched, NULL will be read for the rdev, and data will be lost if step 4 failed. The new synchronization is similar to blk_mq_freeze_queue(). To add or remove disk: 1. Suspend the array, that is, stop new IO from being dispatched and wait for inflight IO to finish. 2. Add or remove rdevs to array; 3. Resume the array; IO path doesn't need to change for now, and all rcu implementation can be removed. Then main work is divided into 3 steps: First, first make sure new apis to suspend the array is general: - make sure suspend array will wait for io to be done(Done by [1]); - make sure suspend array can be called for all personalities(Done by [2]); - make sure suspend array can be called at any time(Done by [3]); - make sure suspend array doesn't rely on 'reconfig_mutex'(PATCH 3-5); Second replace old apis with new apis(PATCH 6-16). Specifically, the synchronization is changed from: lock reconfig_mutex suspend array make changes resume array unlock reconfig_mutex to: suspend array lock reconfig_mutex make changes unlock reconfig_mutex resume array Finally, for the remain path that involved reconfiguration, suspend the array first(PATCH 11,12, [4] and PATCH 17): Preparatory work: [1] https://lore.kernel.org/all/20230621165110.1498313-1-yukuai1@huaweicloud.com/ [2] https://lore.kernel.org/all/20230628012931.88911-2-yukuai1@huaweicloud.com/ [3] https://lore.kernel.org/all/20230825030956.1527023-1-yukuai1@huaweicloud.com/ [4] https://lore.kernel.org/all/20230825031622.1530464-1-yukuai1@huaweicloud.com/ * md-suspend-rewrite: md: rename __mddev_suspend/resume() back to mddev_suspend/resume() md: remove old apis to suspend the array md: suspend array in md_start_sync() if array need reconfiguration md/raid5: replace suspend with quiesce() callback md/md-linear: cleanup linear_add() md: cleanup mddev_create/destroy_serial_pool() md: use new apis to suspend array before mddev_create/destroy_serial_pool md: use new apis to suspend array for ioctls involed array reconfiguration md: use new apis to suspend array for adding/removing rdev from state_store() md: use new apis to suspend array for sysfs apis md/raid5: use new apis to suspend array md/raid5-cache: use new apis to suspend array md/md-bitmap: use new apis to suspend array for location_store() md/dm-raid: use new apis to suspend array md: add new helpers to suspend/resume and lock/unlock array md: add new helpers to suspend/resume array md: replace is_md_suspended() with 'mddev->suspended' in md_check_recovery() md/raid5-cache: use READ_ONCE/WRITE_ONCE for 'conf->log' md: use READ_ONCE/WRITE_ONCE for 'suspend_lo' and 'suspend_hi'
Linux kernel
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