On Thu, Feb 09 2017, Jan Kara wrote: > Commit 6cd18e711dd8 "block: destroy bdi before blockdev is > unregistered." moved bdi unregistration (at that time through > bdi_destroy()) from blk_release_queue() to blk_cleanup_queue() because > it needs to happen before blk_unregister_region() call in del_gendisk() > for MD. As much as it is fine for device registration / unregistration > purposes, it does not fit our needs wrt writeback code. For those we > will need bdi_unregister() to happen after bdev_unhash_inode() so that > we are sure bdev inode is destroyed or soon to be destroyed (as soon as > last inode reference is dropped and nobody should be holding bdev inode > reference for long at this point) because bdi_unregister() may block > waiting for bdev's inode i_wb reference to be dropped and that happens > only once bdev inode gets destroyed. > > Also SCSI will free up the device number from sd_remove() called through > a maze of callbacks from device_del() in __scsi_remove_device() before > blk_cleanup_queue() and thus similar races as described in 6cd18e711dd8 > can happen for SCSI as well as reported by Omar [1]. Moving > bdi_unregister() to del_gendisk() fixes these problems as well since > del_gendisk() gets called from sd_remove() before freeing the device > number. > > This also makes device_add_disk() (calling bdi_register_owner()) more > symmetric with del_gendisk(). What an amazingly sensible idea!! I like that a lot. Thanks, NeilBrown
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