The cleanup_srcu_struct_quiesced() function was added because NVME
used WQ_MEM_RECLAIM workqueues and SRCU did not, which meant that
NVME workqueues waiting on SRCU workqueues could result in deadlocks
during low-memory conditions. However, SRCU now also has WQ_MEM_RECLAIM
workqueues, so there is no longer a potential for deadlock. Furthermore,
it turns out to be extremely hard to use cleanup_srcu_struct_quiesced()
correctly due to the fact that SRCU callback invocation accesses the
srcu_struct structure's per-CPU data area just after callbacks are
invoked. Therefore, the usual practice of using srcu_barrier() to wait
for callbacks to be invoked before invoking cleanup_srcu_struct_quiesced()
fails because SRCU's callback-invocation workqueue handler might be
delayed, which can result in cleanup_srcu_struct_quiesced() being invoked
(and thus freeing the per-CPU data) before the SRCU's callback-invocation
workqueue handler is finished using that per-CPU data. Nor is this a
theoretical problem: KASAN emitted use-after-free warnings because of
this problem on actual runs.
In short, NVME can now safely invoke cleanup_srcu_struct(), which
avoids the use-after-free scenario. And cleanup_srcu_struct_quiesced()
is quite difficult to use safely. This commit therefore removes
cleanup_srcu_struct_quiesced(), switching its sole user back to
cleanup_srcu_struct(). This effectively reverts the following pair
of commits:
f7194ac32c ("srcu: Add cleanup_srcu_struct_quiesced()")
4317228ad9 ("nvme: Avoid flush dependency in delete controller flow")
Reported-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Tested-by: Bart Van Assche <bvanassche@acm.org>
f5ad399149