srcu: Remove cleanup_srcu_struct_quiesced()

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:

f7194ac32ca2 ("srcu: Add cleanup_srcu_struct_quiesced()")
4317228ad9b8 ("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>

1 files changed, 12 insertions, 18 deletions

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 4f30f3ecabc1..9b761e546de8 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -360,8 +360,14 @@ static unsigned long srcu_get_delay(struct srcu_struct *ssp)
 	return SRCU_INTERVAL;
 }
 
-/* Helper for cleanup_srcu_struct() and cleanup_srcu_struct_quiesced(). */
-void _cleanup_srcu_struct(struct srcu_struct *ssp, bool quiesced)
+/**
+ * cleanup_srcu_struct - deconstruct a sleep-RCU structure
+ * @ssp: structure to clean up.
+ *
+ * Must invoke this after you are finished using a given srcu_struct that
+ * was initialized via init_srcu_struct(), else you leak memory.
+ */
+void cleanup_srcu_struct(struct srcu_struct *ssp)
 {
 	int cpu;
 
@@ -369,24 +375,12 @@ void _cleanup_srcu_struct(struct srcu_struct *ssp, bool quiesced)
 		return; /* Just leak it! */
 	if (WARN_ON(srcu_readers_active(ssp)))
 		return; /* Just leak it! */
-	if (quiesced) {
-		if (WARN_ON(delayed_work_pending(&ssp->work)))
-			return; /* Just leak it! */
-	} else {
-		flush_delayed_work(&ssp->work);
-	}
+	flush_delayed_work(&ssp->work);
 	for_each_possible_cpu(cpu) {
 		struct srcu_data *sdp = per_cpu_ptr(ssp->sda, cpu);
 
-		if (quiesced) {
-			if (WARN_ON(timer_pending(&sdp->delay_work)))
-				return; /* Just leak it! */
-			if (WARN_ON(work_pending(&sdp->work)))
-				return; /* Just leak it! */
-		} else {
-			del_timer_sync(&sdp->delay_work);
-			flush_work(&sdp->work);
-		}
+		del_timer_sync(&sdp->delay_work);
+		flush_work(&sdp->work);
 		if (WARN_ON(rcu_segcblist_n_cbs(&sdp->srcu_cblist)))
 			return; /* Forgot srcu_barrier(), so just leak it! */
 	}
@@ -399,7 +393,7 @@ void _cleanup_srcu_struct(struct srcu_struct *ssp, bool quiesced)
 	free_percpu(ssp->sda);
 	ssp->sda = NULL;
 }
-EXPORT_SYMBOL_GPL(_cleanup_srcu_struct);
+EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
 
 /*
  * Counts the new reader in the appropriate per-CPU element of the