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* [PATCH v2] btrfs: fix rw device counting in __btrfs_free_extra_devids
@ 2021-07-27  7:13 Desmond Cheong Zhi Xi
  2021-07-28 12:58 ` David Sterba
  2021-08-12 10:38 ` David Sterba
  0 siblings, 2 replies; 14+ messages in thread
From: Desmond Cheong Zhi Xi @ 2021-07-27  7:13 UTC (permalink / raw)
  To: clm, josef, dsterba
  Cc: Desmond Cheong Zhi Xi, anand.jain, linux-btrfs, linux-kernel,
	skhan, gregkh, linux-kernel-mentees, syzbot+a70e2ad0879f160b9217

When removing a writeable device in __btrfs_free_extra_devids, the rw
device count should be decremented.

This error was caught by Syzbot which reported a warning in
close_fs_devices because fs_devices->rw_devices was not 0 after
closing all devices. Here is the call trace that was observed:

      device_list_add();   <---------------- device added
        btrfs_open_one_device();   <-------- writable device opened,
	                                     rw device count ++
	  __btrfs_free_extra_devids();  <--- writable device removed,
	                              rw device count not decremented
	      close_fs_devices();   <------- rw device count off by 1

As a note, prior to commit cf89af146b7e ("btrfs: dev-replace: fail
mount if we don't have replace item with target device"), rw_devices
was decremented on removing a writable device in
__btrfs_free_extra_devids only if the BTRFS_DEV_STATE_REPLACE_TGT bit
was not set for the device. However, this check does not need to be
reinstated as it is now redundant and incorrect.

In __btrfs_free_extra_devids, we skip removing the device if it is the
target for replacement. This is done by checking whether device->devid
only on the device with devid BTRFS_DEV_REPLACE_DEVID, no devices
should have the BTRFS_DEV_STATE_REPLACE_TGT bit set after the check,
and so it's redundant to test for that bit.

Additionally, following commit 82372bc816d7 ("Btrfs: make
the logic of source device removing more clear"), rw_devices is
incremented whenever a writeable device is added to the alloc
list (including the target device in btrfs_dev_replace_finishing), so
all removals of writable devices from the alloc list should also be
accompanied by a decrement to rw_devices.

Fixes: cf89af146b7e ("btrfs: dev-replace: fail mount if we don't have replace item with target device")
Signed-off-by: Desmond Cheong Zhi Xi <>
Reviewed-by: Anand Jain <>
 fs/btrfs/volumes.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 807502cd6510..916c25371658 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -1078,6 +1078,7 @@ static void __btrfs_free_extra_devids(struct btrfs_fs_devices *fs_devices,
 		if (test_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state)) {
 			clear_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state);
+			fs_devices->rw_devices--;

^ permalink raw reply	[flat|nested] 14+ messages in thread

end of thread, other threads:[~2021-08-20 17:53 UTC | newest]

Thread overview: 14+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-07-27  7:13 [PATCH v2] btrfs: fix rw device counting in __btrfs_free_extra_devids Desmond Cheong Zhi Xi
2021-07-28 12:58 ` David Sterba
2021-08-12 10:38 ` David Sterba
2021-08-12 15:43   ` Desmond Cheong Zhi Xi
2021-08-12 15:50     ` David Sterba
2021-08-12 17:31       ` Desmond Cheong Zhi Xi
2021-08-13  8:51         ` David Sterba
2021-08-13  9:57           ` Desmond Cheong Zhi Xi
2021-08-13 10:30             ` David Sterba
2021-08-19 17:11               ` Desmond Cheong Zhi Xi
2021-08-19 17:34                 ` David Sterba
2021-08-20  3:09                   ` Desmond Cheong Zhi Xi
2021-08-20 10:58                     ` David Sterba
2021-08-20 17:53                       ` Desmond Cheong Zhi Xi

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