On Tue, Oct 10, 2017 at 03:51:03PM -0600, Liu Bo wrote: > It's pointless to defer it to a kthread helper as we're not under any > special context.
I agree the doubly deferred freeing is pointless. It's a weird mix of RCU and workques and understanding all the interactions turned out to be hard, last time I tried. The RCU stuff needs the rcu_barriers, and the callback can be served from any process context. While the workqueus have their dedicated kthreads. Calling free_device() is quick, it just adds the work to the queue and returns. This makes __btrfs_close_devices/btrfs_rm_device/... and all other callers fast, at the cost that there must be some explicit barrier or waiting done when we want to make sure all the device resources have been freed. I can't say the quick return is wrong, but it makes the device lifetime hard to understand. The device freeing callback (__free_device) is lightweight, but also calls "rcu_free" for the device name. I have WIP patches to clean up the rcu and locking around devices and actually document the rules, but with unreviewed pile in the mailinglist I can't tell when this is going to land. If you want to simplify at least the device freeing, please go on, and explain in the changelog that it's not going to break anything. The hand-wavy sentence is not what I'd expect :) -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
