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Date:   Mon, 7 Sep 2020 10:04:32 +1000
From:   Dave Chinner <david@fromorbit.com>
To:     Christoph Hellwig <hch@lst.de>
Cc:     Josef Bacik <josef@toxicpanda.com>,
        Johannes Thumshirn <johannes.thumshirn@wdc.com>,
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        Filipe Manana <fdmanana@gmail.com>,
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Subject: Re: [RFC PATCH] btrfs: don't call btrfs_sync_file from iomap context
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On Thu, Sep 03, 2020 at 06:32:36PM +0200, Christoph Hellwig wrote:
> We could trivially do something like this to allow the file system
> to call iomap_dio_complete without i_rwsem:

That just exposes another deadlock vector:

P0			P1
inode_lock()		fallocate(FALLOC_FL_ZERO_RANGE)
__iomap_dio_rw()	inode_lock()
			<block>
<submits IO>
<completes IO>
inode_unlock()
			<gets inode_lock()>
			inode_dio_wait()
iomap_dio_complete()
  generic_write_sync()
    btrfs_file_fsync()
      inode_lock()
      <deadlock>

Basically, the only safe thing to do is implement ->fsync without
holding the DIO IO submission lock....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com