[PATCH 2/2] closures: Fix race in closure_sync()

[Kent Overstreet <kent.overstreet@linux.dev>]

As pointed out by Linus, closure_sync() was racy; we could skip blocking
immediately after a get() and a put(), but then that would skip any
barrier corresponding to the other thread's put() barrier.

To fix this, always do the full __closure_sync() sequence whenever any
get() has happened and the closure might have been used by other
threads.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
---
 fs/bcachefs/fs-io-direct.c |  1 +
 include/linux/closure.h    | 10 +++++++++-
 lib/closure.c              |  3 +++
 3 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/fs/bcachefs/fs-io-direct.c b/fs/bcachefs/fs-io-direct.c
index 6a9557e7ecab..5b42a76c4796 100644
--- a/fs/bcachefs/fs-io-direct.c
+++ b/fs/bcachefs/fs-io-direct.c
@@ -113,6 +113,7 @@ static int bch2_direct_IO_read(struct kiocb *req, struct iov_iter *iter)
 	} else {
 		atomic_set(&dio->cl.remaining,
 			   CLOSURE_REMAINING_INITIALIZER + 1);
+		dio->cl.closure_get_happened = true;
 	}
 
 	dio->req	= req;
diff --git a/include/linux/closure.h b/include/linux/closure.h
index bdab17050bc8..de7bb47d8a46 100644
--- a/include/linux/closure.h
+++ b/include/linux/closure.h
@@ -154,6 +154,7 @@ struct closure {
 	struct closure		*parent;
 
 	atomic_t		remaining;
+	bool			closure_get_happened;
 
 #ifdef CONFIG_DEBUG_CLOSURES
 #define CLOSURE_MAGIC_DEAD	0xc054dead
@@ -185,7 +186,11 @@ static inline unsigned closure_nr_remaining(struct closure *cl)
  */
 static inline void closure_sync(struct closure *cl)
 {
-	if (closure_nr_remaining(cl) != 1)
+#ifdef CONFIG_DEBUG_CLOSURES
+	BUG_ON(closure_nr_remaining(cl) != 1 && !cl->closure_get_happened);
+#endif
+
+	if (cl->closure_get_happened)
 		__closure_sync(cl);
 }
 
@@ -257,6 +262,8 @@ static inline void closure_queue(struct closure *cl)
  */
 static inline void closure_get(struct closure *cl)
 {
+	cl->closure_get_happened = true;
+
 #ifdef CONFIG_DEBUG_CLOSURES
 	BUG_ON((atomic_inc_return(&cl->remaining) &
 		CLOSURE_REMAINING_MASK) <= 1);
@@ -279,6 +286,7 @@ static inline void closure_init(struct closure *cl, struct closure *parent)
 		closure_get(parent);
 
 	atomic_set(&cl->remaining, CLOSURE_REMAINING_INITIALIZER);
+	cl->closure_get_happened = false;
 
 	closure_debug_create(cl);
 	closure_set_ip(cl);
diff --git a/lib/closure.c b/lib/closure.c
index 501dfa277b59..f86c9eeafb35 100644
--- a/lib/closure.c
+++ b/lib/closure.c
@@ -23,6 +23,8 @@ static inline void closure_put_after_sub(struct closure *cl, int flags)
 	if (!r) {
 		smp_acquire__after_ctrl_dep();
 
+		cl->closure_get_happened = false;
+
 		if (cl->fn && !(flags & CLOSURE_DESTRUCTOR)) {
 			atomic_set(&cl->remaining,
 				   CLOSURE_REMAINING_INITIALIZER);
@@ -92,6 +94,7 @@ bool closure_wait(struct closure_waitlist *waitlist, struct closure *cl)
 	if (atomic_read(&cl->remaining) & CLOSURE_WAITING)
 		return false;
 
+	cl->closure_get_happened = true;
 	closure_set_waiting(cl, _RET_IP_);
 	atomic_add(CLOSURE_WAITING + 1, &cl->remaining);
 	llist_add(&cl->list, &waitlist->list);
-- 
2.42.0