Re: [PATCH v3] btrfs: fix a possible umount deadlock

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On Thu, Sep 22, 2016 at 12:24:07PM -0400, Jeff Mahoney wrote:
> This isn't necessarily a comment on this code in particular, but what's
> the reason for using call_rcu to defer the freeing instead of
> synchronize_rcu and freeing the device directly via __free_device (if it
> accepted a btrfs_device)?  It's a pattern that's used throughout
> volumes.c and it's old[1].  I'm not sure if it was intentional to defer
> freeing since that was actually a functional change from the code it
> replaced.

It could be unintentional, the patch was supposed to relax the read-only
side, ie. use RCU instead of mutex. Deferred freeing does not seem to be
necessary. The device locking code needs cleanup/refactoring.
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