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>

5 files changed, 18 insertions, 66 deletions

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index 470601980794..739c5b4830d7 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -388,7 +388,7 @@ static void nvme_free_ns_head(struct kref *ref)
         nvme_mpath_remove_disk(head);
         ida_simple_remove(&head->subsys->ns_ida, head->instance);
         list_del_init(&head->entry);
-        cleanup_srcu_struct_quiesced(&head->srcu);
+        cleanup_srcu_struct(&head->srcu);
         nvme_put_subsystem(head->subsys);
         kfree(head);
 }
diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index c495b2d51569..e432cc92c73d 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -56,45 +56,11 @@ struct srcu_struct { };
 
 void call_srcu(struct srcu_struct *ssp, struct rcu_head *head,
                 void (*func)(struct rcu_head *head));
-void _cleanup_srcu_struct(struct srcu_struct *ssp, bool quiesced);
+void cleanup_srcu_struct(struct srcu_struct *ssp);
 int __srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp);
 void __srcu_read_unlock(struct srcu_struct *ssp, int idx) __releases(ssp);
 void synchronize_srcu(struct srcu_struct *ssp);
 
-/**
- * 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.
- */
-static inline void cleanup_srcu_struct(struct srcu_struct *ssp)
-{
-        _cleanup_srcu_struct(ssp, false);
-}
-
-/**
- * cleanup_srcu_struct_quiesced - deconstruct a quiesced 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.  Also,
- * all grace-period processing must have completed.
- *
- * "Completed" means that the last synchronize_srcu() and
- * synchronize_srcu_expedited() calls must have returned before the call
- * to cleanup_srcu_struct_quiesced().  It also means that the callback
- * from the last call_srcu() must have been invoked before the call to
- * cleanup_srcu_struct_quiesced(), but you can use srcu_barrier() to help
- * with this last.  Violating these rules will get you a WARN_ON() splat
- * (with high probability, anyway), and will also cause the srcu_struct
- * to be leaked.
- */
-static inline void cleanup_srcu_struct_quiesced(struct srcu_struct *ssp)
-{
-        _cleanup_srcu_struct(ssp, true);
-}
-
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 
 /**
diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index f14d1b18a74f..d2b226110835 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -592,12 +592,7 @@ static void srcu_torture_init(void)
 
 static void srcu_torture_cleanup(void)
 {
-        static DEFINE_TORTURE_RANDOM(rand);
-
-        if (torture_random(&rand) & 0x800)
-                cleanup_srcu_struct(&srcu_ctld);
-        else
-                cleanup_srcu_struct_quiesced(&srcu_ctld);
+        cleanup_srcu_struct(&srcu_ctld);
         srcu_ctlp = &srcu_ctl; /* In case of a later rcutorture run. */
 }
 
diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
index 5d4a39a6505a..44d6606b8325 100644
--- a/kernel/rcu/srcutiny.c
+++ b/kernel/rcu/srcutiny.c
@@ -76,19 +76,16 @@ EXPORT_SYMBOL_GPL(init_srcu_struct);
  * 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, bool quiesced)
+void cleanup_srcu_struct(struct srcu_struct *ssp)
 {
         WARN_ON(ssp->srcu_lock_nesting[0] || ssp->srcu_lock_nesting[1]);
-        if (quiesced)
-                WARN_ON(work_pending(&ssp->srcu_work));
-        else
-                flush_work(&ssp->srcu_work);
+        flush_work(&ssp->srcu_work);
         WARN_ON(ssp->srcu_gp_running);
         WARN_ON(ssp->srcu_gp_waiting);
         WARN_ON(ssp->srcu_cb_head);
         WARN_ON(&ssp->srcu_cb_head != ssp->srcu_cb_tail);
 }
-EXPORT_SYMBOL_GPL(_cleanup_srcu_struct);
+EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
 
 /*
  * Removes the count for the old reader from the appropriate element of
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