linux-kernel - [PATCH 5.4 032/118] rbd: avoid a deadlock on header

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Message-Id: <20200422095037.117674327@linuxfoundation.org>
Date:   Wed, 22 Apr 2020 11:56:33 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Ilya Dryomov <idryomov@...il.com>,
        Jason Dillaman <dillaman@...hat.com>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.4 032/118] rbd: avoid a deadlock on header_rwsem when flushing notifies

From: Ilya Dryomov <idryomov@...il.com>

[ Upstream commit 0e4e1de5b63fa423b13593337a27fd2d2b0bcf77 ]

rbd_unregister_watch() flushes notifies and therefore cannot be called
under header_rwsem because a header update notify takes header_rwsem to
synchronize with "rbd map".  If mapping an image fails after the watch
is established and a header update notify sneaks in, we deadlock when
erroring out from rbd_dev_image_probe().

Move watch registration and unregistration out of the critical section.
The only reason they were put there was to make header_rwsem management
slightly more obvious.

Fixes: 811c66887746 ("rbd: fix rbd map vs notify races")
Signed-off-by: Ilya Dryomov <idryomov@...il.com>
Reviewed-by: Jason Dillaman <dillaman@...hat.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 drivers/block/rbd.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
index a67315786db47..bab9c546ba334 100644
--- a/drivers/block/rbd.c
+++ b/drivers/block/rbd.c
@@ -4636,6 +4636,10 @@ static void cancel_tasks_sync(struct rbd_device *rbd_dev)
 	cancel_work_sync(&rbd_dev->unlock_work);
 }
 
+/*
+ * header_rwsem must not be held to avoid a deadlock with
+ * rbd_dev_refresh() when flushing notifies.
+ */
 static void rbd_unregister_watch(struct rbd_device *rbd_dev)
 {
 	cancel_tasks_sync(rbd_dev);
@@ -6942,6 +6946,9 @@ static void rbd_dev_image_release(struct rbd_device *rbd_dev)
  * device.  If this image is the one being mapped (i.e., not a
  * parent), initiate a watch on its header object before using that
  * object to get detailed information about the rbd image.
+ *
+ * On success, returns with header_rwsem held for write if called
+ * with @depth == 0.
  */
 static int rbd_dev_image_probe(struct rbd_device *rbd_dev, int depth)
 {
@@ -6974,6 +6981,9 @@ static int rbd_dev_image_probe(struct rbd_device *rbd_dev, int depth)
 		}
 	}
 
+	if (!depth)
+		down_write(&rbd_dev->header_rwsem);
+
 	ret = rbd_dev_header_info(rbd_dev);
 	if (ret)
 		goto err_out_watch;
@@ -7027,6 +7037,8 @@ static int rbd_dev_image_probe(struct rbd_device *rbd_dev, int depth)
 err_out_probe:
 	rbd_dev_unprobe(rbd_dev);
 err_out_watch:
+	if (!depth)
+		up_write(&rbd_dev->header_rwsem);
 	if (!depth)
 		rbd_unregister_watch(rbd_dev);
 err_out_format:
@@ -7085,12 +7097,9 @@ static ssize_t do_rbd_add(struct bus_type *bus,
 		goto err_out_rbd_dev;
 	}
 
-	down_write(&rbd_dev->header_rwsem);
 	rc = rbd_dev_image_probe(rbd_dev, 0);
-	if (rc < 0) {
-		up_write(&rbd_dev->header_rwsem);
+	if (rc < 0)
 		goto err_out_rbd_dev;
-	}
 
 	/* If we are mapping a snapshot it must be marked read-only */
 	if (rbd_dev->spec->snap_id != CEPH_NOSNAP)
-- 
2.20.1