fs/aio: Add explicit RCU grace period when freeing kioctx

fs/aio: Add explicit RCU grace period when freeing kioctx

While fixing refcounting, e34ecee2ae79 ("aio: Fix a trinity splat")
incorrectly removed explicit RCU grace period before freeing kioctx.
The intention seems to be depending on the internal RCU grace periods
of percpu_ref; however, percpu_ref uses a different flavor of RCU,
sched-RCU.  This can lead to kioctx being freed while RCU read
protected dereferences are still in progress.

Fix it by updating free_ioctx() to go through call_rcu() explicitly.

v2: Comment added to explain double bouncing.

Signed-off-by: Tejun Heo <[email protected]>
Reported-by: Jann Horn <[email protected]>
Fixes: e34ecee2ae79 ("aio: Fix a trinity splat")
Cc: Kent Overstreet <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: [email protected] # v3.13+