linux-kernel - [PATCH v1] virtio-mem: fix sleeping in RCU read side section in virtio_mem_online_page

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Date:   Wed, 25 Aug 2021 12:24:15 +0200
From:   David Hildenbrand <david@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     David Hildenbrand <david@...hat.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        virtualization@...ts.linux-foundation.org
Subject: [PATCH v1] virtio-mem: fix sleeping in RCU read side section in virtio_mem_online_page_cb()

virtio_mem_set_fake_offline() might sleep now, and we call it under
rcu_read_lock(). To fix it, simply move the rcu_read_unlock() further
up, as we're done with the device.

Reported-by: Dan Carpenter <dan.carpenter@...cle.com>
Fixes: 6cc26d77613a: "virtio-mem: use page_offline_(start|end) when setting PageOffline()
Cc: "Michael S. Tsirkin" <mst@...hat.com>
Cc: Jason Wang <jasowang@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: virtualization@...ts.linux-foundation.org
Signed-off-by: David Hildenbrand <david@...hat.com>
---

The problematic commit is in v5.14-rc1 .. v5.14-rc7, but it suspect
might be too late for v5.14.

The original commit went upstream via Andrews tree, we could take this fix
via Andrews tree as well or via the vhost tree (MST), I don't particularly
care. (putting Linus on CC just in case)

---
 drivers/virtio/virtio_mem.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/virtio/virtio_mem.c b/drivers/virtio/virtio_mem.c
index 09ed55de07d7..b91bc810a87e 100644
--- a/drivers/virtio/virtio_mem.c
+++ b/drivers/virtio/virtio_mem.c
@@ -1242,12 +1242,19 @@ static void virtio_mem_online_page_cb(struct page *page, unsigned int order)
 			do_online = virtio_mem_bbm_get_bb_state(vm, id) !=
 				    VIRTIO_MEM_BBM_BB_FAKE_OFFLINE;
 		}
+
+		/*
+		 * virtio_mem_set_fake_offline() might sleep, we don't need
+		 * the device anymore. See virtio_mem_remove() how races
+		 * between memory onlining and device removal are handled.
+		 */
+		rcu_read_unlock();
+
 		if (do_online)
 			generic_online_page(page, order);
 		else
 			virtio_mem_set_fake_offline(PFN_DOWN(addr), 1 << order,
 						    false);
-		rcu_read_unlock();
 		return;
 	}
 	rcu_read_unlock();
-- 
2.31.1