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726a9b67e9
From Yu Kuai:
md: remove rcu protection to access rdev from conf
The lifetime of rdev:
1. md_import_device() generate a rdev based on underlying disk;
mddev_lock()
rdev = kzalloc();
rdev->bdev = blkdev_get_by_dev();
mddev_unlock()
2. bind_rdev_to_array() add this rdev to mddev->disks;
mddev_lock()
kobject_add(&rdev->kobj, &mddev->kobj, ...);
list_add_rcu(&rdev->same_set, &mddev->disks);
mddev_unlock()
3. remove_and_add_spares() add this rdev to conf;
mddev_lock()
rdev_addable();
pers->hot_add_disk();
rcu_assign_pointer(conf->rdev, rdev);
mddev_unlock()
4. Use this array with rdev;
5. remove_and_add_spares() remove rdev from conf;
// triggered by sysfs/ioctl
mddev_lock()
rdev_removeable();
pers->hot_remove_disk();
rcu_assign_pointer(conf->rdev, NULL);
synchronize_rcu();
mddev_unlock()
// triggered by daemon
mddev_lock()
rdev_removeable();
synchronize_rcu(); -> this can't protect accessing rdev from conf
pers->hot_remove_disk();
rcu_assign_pointer(conf->rdev, NULL);
mddev_unlock()
6. md_kick_rdev_from_array() remove rdev from mddev->disks;
mddev_lock()
list_del_rcu(&rdev->same_set);
synchronize_rcu();
list_add(&rdev->same_set, &mddev->deleting)
mddev_unlock()
export_rdev
There are two separate rcu protection for rdev, and this pathset remove
the protection of conf(step 3 and 5), because it's safe to access rdev
from conf in following cases:
- If 'reconfig_mutex' is held, because rdev can't be added or rmoved to
conf;
- If there is normal IO inflight, because mddev_suspend() will wait for
IO to be done and prevent rdev to be added or removed to conf;
- If sync thread is running, because remove_and_add_spares() can only be
called from daemon thread when sync thread is done, and
'MD_RECOVERY_RUNNING' is also checked for ioctl/sysfs;
- if any spinlock or rcu_read_lock() is held, because synchronize_rcu()
from step 6 prevent rdev to be freed until spinlock is released or
rcu_read_unlock();
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