Re: [PATCH] Btrfs: fix block group remaining RO forever after error during device replace

[David Sterba <dsterba@suse.cz>]

On Thu, Nov 14, 2019 at 06:02:43PM +0000, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> When doing a device replace, while at scrub.c:scrub_enumerate_chunks(), we
> set the block group to RO mode and then wait for any ongoing writes into
> extents of the block group to complete. While doing that wait we overwrite
> the value of the variable 'ret' and can break out of the loop if an error
> happens without turning the block group back into RW mode. So what happens
> is the following:
> 
> 1) btrfs_inc_block_group_ro() returns 0, meaning it set the block group
>    to RO mode (its ->ro field set to 1 or incremented to some value > 1);
> 
> 2) Then btrfs_wait_ordered_roots() returns a value > 0;
> 
> 3) Then if either joinning or committing the transaction fails, we break
>    out of the loop wihtout calling btrfs_dec_block_group_ro(), leaving
>    the block group in RO mode forever.
> 
> To fix this, just remove the code that waits for ongoing writes to extents
> of the block group, since it's not needed because in the initial setup
> phase of a device replace operation, before starting to find all chunks
> and their extents, we set the target device for replace while holding
> fs_info->dev_replace->rwsem, which ensures that after releasing that
> semaphore, any writes into the source device are made to the target device
> as well (__btrfs_map_block() guarantees that). So while at
> scrub_enumerate_chunks() we only need to worry about finding and copying
> extents (from the source device to the target device) that were written
> before we started the device replace operation.
> 
> Fixes: f0e9b7d6401959 ("Btrfs: fix race setting block group readonly during device replace")
> Signed-off-by: Filipe Manana <fdmanana@suse.com>

Added to misc-next, thanks.