Re: [PATCH 04/15] iomap: Call inode_dio_end() before generic_write_sync()

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Tue, Sep 22, 2020 at 10:20:11AM -0400, Josef Bacik wrote:
> On 9/21/20 10:43 AM, Goldwyn Rodrigues wrote:
> > From: Goldwyn Rodrigues <rgoldwyn@suse.com>
> > 
> > iomap complete routine can deadlock with btrfs_fallocate because of the
> > call to generic_write_sync().
> > 
> > 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>
> > 
> > inode_dio_end() is used to notify the end of DIO data in order
> > to synchronize with truncate. Call inode_dio_end() before calling
> > generic_write_sync(), so filesystems can lock i_rwsem during a sync.
> > 
> > ---
> >   fs/iomap/direct-io.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> > index d970c6bbbe11..e01f81e7b76f 100644
> > --- a/fs/iomap/direct-io.c
> > +++ b/fs/iomap/direct-io.c
> > @@ -118,6 +118,7 @@ ssize_t iomap_dio_complete(struct iomap_dio *dio)
> >   			dio_warn_stale_pagecache(iocb->ki_filp);
> >   	}
> > +	inode_dio_end(file_inode(iocb->ki_filp));
> >   	/*
> >   	 * If this is a DSYNC write, make sure we push it to stable storage now
> >   	 * that we've written data.
> > @@ -125,7 +126,6 @@ ssize_t iomap_dio_complete(struct iomap_dio *dio)
> >   	if (ret > 0 && (dio->flags & IOMAP_DIO_NEED_SYNC))
> >   		ret = generic_write_sync(iocb, ret);
> > -	inode_dio_end(file_inode(iocb->ki_filp));
> >   	kfree(dio);
> >   	return ret;
> > 
> 
> Did you verify that xfs or ext4 don't rely on the inode_dio_end() happening
> before the generic_write_sync()?  I wouldn't expect that they would, but
> we've already run into problems making those kind of assumptions.  If it's
> fine you can add

I was gonna ask the same question, but as there's no SoB on this patch I
hadn't really looked at it yet. ;)

Operations that rely on inode_dio_wait to have blocked until all the
directios are complete could get tripped up by iomap not having done the
generic_write_sync to stabilise the metadata, but I /think/ most
operations that do that also themselves flush the file.  But I don't
really know if there's a subtlety there if the inode_dio_wait thread
manages to grab the ILOCK before the generic_write_sync thread does.

--D

> Reviewed-by: Josef Bacik <josef@toxicpanda.com>
> 
> Thanks,
> 
> Josef