drbd: make suspend_io() / resume_io() must be thread and recursion safe

Avoid to prematurely resume application IO: don't set/clear a single
bit, but inc/dec an atomic counter.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/block/drbd/drbd_int.h

@@ -500,7 +500,6 @@ enum {
 
 	MD_NO_FUA,		/* Users wants us to not use FUA/FLUSH on meta data dev */
 
-	SUSPEND_IO,		/* suspend application io */
 	BITMAP_IO,		/* suspend application io;
 				   once no more io in flight, start bitmap io */
 	BITMAP_IO_QUEUED,       /* Started bitmap IO */
@@ -880,6 +879,7 @@ struct drbd_device {
 	atomic_t rs_pending_cnt; /* RS request/data packets on the wire */
 	atomic_t unacked_cnt;	 /* Need to send replies for */
 	atomic_t local_cnt;	 /* Waiting for local completion */
+	atomic_t suspend_cnt;
 
 	/* Interval tree of pending local requests */
 	struct rb_root read_requests;
@@ -2263,7 +2263,7 @@ static inline bool may_inc_ap_bio(struct drbd_device *device)
 
 	if (drbd_suspended(device))
 		return false;
-	if (test_bit(SUSPEND_IO, &device->flags))
+	if (atomic_read(&device->suspend_cnt))
 		return false;
 
 	/* to avoid potential deadlock or bitmap corruption,

drivers/block/drbd/drbd_nl.c

@@ -865,9 +865,11 @@ char *ppsize(char *buf, unsigned long long size)
  * and can be long lived.
  * This changes an device->flag, is triggered by drbd internals,
  * and should be short-lived. */
+/* It needs to be a counter, since multiple threads might
+   independently suspend and resume IO. */
 void drbd_suspend_io(struct drbd_device *device)
 {
-	set_bit(SUSPEND_IO, &device->flags);
+	atomic_inc(&device->suspend_cnt);
 	if (drbd_suspended(device))
 		return;
 	wait_event(device->misc_wait, !atomic_read(&device->ap_bio_cnt));
@@ -875,7 +877,7 @@ void drbd_suspend_io(struct drbd_device *device)
 
 void drbd_resume_io(struct drbd_device *device)
 {
-	clear_bit(SUSPEND_IO, &device->flags);
+	if (atomic_dec_and_test(&device->suspend_cnt))
 		wake_up(&device->misc_wait);
 }
 

drivers/block/drbd/drbd_state.c

@@ -1484,7 +1484,7 @@ int drbd_bitmap_io_from_worker(struct drbd_device *device,
 	D_ASSERT(device, current == first_peer_device(device)->connection->worker.task);
 
 	/* open coded non-blocking drbd_suspend_io(device); */
-	set_bit(SUSPEND_IO, &device->flags);
+	atomic_inc(&device->suspend_cnt);
 
 	drbd_bm_lock(device, why, flags);
 	rv = io_fn(device);