nvmet: don't allow I/O admission after percpu ns reference is killed

nvmet_req_find_ns() uses percpu_ref_get() to obtain a reference to
the namespace. However, percpu_ref_get() can acquire a reference even
after the namespace reference has been killed (or marked DEAD).

This is undesirable during namespace disable because nvmet_ns_disable()
kills the namespace reference and then waits for all outstanding
references to drain. Acquiring a new reference after the reference is
killed can therefore extend the namespace drain period.

Replace percpu_ref_get() in nvmet_req_find_ns() with
percpu_ref_tryget_live_rcu(), which only acquires a reference while
the namespace reference is still live. This handles the race where
nvmet_req_find_ns() finds ns is admitting I/O (or it's live) but before
it acquires the reference to ns, its reference is killed in
nvmet_ns_disable(). For instance check this race:

CPU0                              CPU1
nvmet_req_find_ns():              nvmet_ns_disable():
xa_load() -> ns
IO_LIVE == set

                                  xa_clear_mark()
                                  percpu_ref_kill() // DEAD

percpu_ref_get()                  synchronize_rcu()
        |                         wait_for_completion()
        +-- succeed

Replacing percpu_ref_get() with percpu_ref_tryget_live_rcu() prevents
the I/O request from acquiring a namespace reference once the
reference has been marked DEAD.

Perform the namespace lookup and reference acquisition in
nvmet_req_find_ns() within an RCU read-side critical section.
nvmet_ns_disable() uses synchronize_rcu() before draining and exiting
the namespace reference, ensuring that RCU readers which may be
acquiring the namespace reference have completed before the reference
is exited.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>

0 files changed, 0 insertions, 0 deletions