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From: Christoph Hellwig <hch@lst.de>
To: Dave Chinner <david@fromorbit.com>
Cc: Goldwyn Rodrigues <rgoldwyn@suse.com>,
	Christoph Hellwig <hch@lst.de>,
	Josef Bacik <josef@toxicpanda.com>,
	Johannes Thumshirn <johannes.thumshirn@wdc.com>,
	David Sterba <dsterba@suse.com>,
	"linux-btrfs @ vger . kernel . org" <linux-btrfs@vger.kernel.org>,
	Filipe Manana <fdmanana@gmail.com>,
	Linux FS Devel <linux-fsdevel@vger.kernel.org>
Subject: Re: [RFC PATCH] btrfs: don't call btrfs_sync_file from iomap context
Date: Thu, 17 Sep 2020 07:52:32 +0200	[thread overview]
Message-ID: <20200917055232.GA31646@lst.de> (raw)
In-Reply-To: <20200917030942.GU12096@dread.disaster.area>

On Thu, Sep 17, 2020 at 01:09:42PM +1000, Dave Chinner wrote:
> > > iomap_dio_complete()
> > >   generic_write_sync()
> > >     btrfs_file_fsync()
> > >       inode_lock()
> > >       <deadlock>
> > 
> > Can inode_dio_end() be called before generic_write_sync(), as it is done
> > in fs/direct-io.c:dio_complete()?
> 
> Don't think so.  inode_dio_wait() is supposed to indicate that all
> DIO is complete, and having the "make it stable" parts of an O_DSYNC
> DIO still running after inode_dio_wait() returns means that we still
> have DIO running....
> 
> For some filesystems, ensuring the DIO data is stable may involve
> flushing other data (perhaps we did EOF zeroing before the file
> extending DIO) and/or metadata to the log, so we need to guarantee
> these DIO related operations are complete and stable before we say
> the DIO is done.

inode_dio_wait really just waits for active I/O that writes to or reads
from the file.  It does not imply that the I/O is stable, just like
i_rwsem itself doesn't.  Various file systems have historically called
the syncing outside i_rwsem and inode_dio_wait (in fact that is what the
fs/direct-io.c code does, so XFS did as well until a few years ago), and
that isn't a problem at all - we just can't return to userspace (or call
ki_complete for in-kernel users) before the data is stable on disk.

  reply	other threads:[~2020-09-17  5:59 UTC|newest]

Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-01 13:06 [RFC PATCH] btrfs: don't call btrfs_sync_file from iomap context Johannes Thumshirn
2020-09-01 13:11 ` Johannes Thumshirn
2020-09-01 14:17 ` Goldwyn Rodrigues
2020-09-01 14:20   ` Johannes Thumshirn
2020-09-01 14:37 ` Filipe Manana
2020-09-01 14:44   ` Johannes Thumshirn
2020-09-01 18:40     ` Goldwyn Rodrigues
2020-09-01 15:11 ` Josef Bacik
2020-09-01 17:45   ` Darrick J. Wong
2020-09-01 17:55     ` Josef Bacik
2020-09-01 21:46   ` Dave Chinner
2020-09-01 22:19     ` Josef Bacik
2020-09-01 23:58       ` Dave Chinner
2020-09-02  0:22         ` Josef Bacik
2020-09-02  7:12           ` Johannes Thumshirn
2020-09-02 11:10             ` Josef Bacik
2020-09-02 16:29               ` Darrick J. Wong
2020-09-02 16:47                 ` Josef Bacik
2020-09-02 11:44         ` Matthew Wilcox
2020-09-02 12:20           ` Dave Chinner
2020-09-02 12:42             ` Josef Bacik
2020-09-03  2:28               ` Dave Chinner
2020-09-03  9:49                 ` Filipe Manana
2020-09-03 16:32   ` Christoph Hellwig
2020-09-03 16:46     ` Josef Bacik
2020-09-07  0:04     ` Dave Chinner
2020-09-15 21:48       ` Goldwyn Rodrigues
2020-09-17  3:09         ` Dave Chinner
2020-09-17  5:52           ` Christoph Hellwig [this message]
2020-09-17  6:29             ` Dave Chinner
2020-09-17  6:42               ` Christoph Hellwig

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