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Re: [PATCH 4/4] xfs: Protect xfs_file_aio_write() with sb_start_write -

To: Jan Kara <jack@xxxxxxx>
Subject: Re: [PATCH 4/4] xfs: Protect xfs_file_aio_write() with sb_start_write - sb_end_write
From: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Date: Thu, 12 Jan 2012 21:29:41 +0000
Cc: linux-fsdevel@xxxxxxxxxxxxxxx, LKML <linux-kernel@xxxxxxxxxxxxxxx>, linux-ext4@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, Eric Sandeen <sandeen@xxxxxxxxxxx>, Dave Chinner <dchinner@xxxxxxxxxx>, Surbhi Palande <csurbhi@xxxxxxxxx>, Kamal Mostafa <kamal@xxxxxxxxxxxxx>, Christoph Hellwig <hch@xxxxxxxxxxxxx>, Ben Myers <bpm@xxxxxxx>, Alex Elder <elder@xxxxxxxxxx>
In-reply-to: <1326331253-6497-5-git-send-email-jack@xxxxxxx>
References: <1326331253-6497-1-git-send-email-jack@xxxxxxx> <1326331253-6497-5-git-send-email-jack@xxxxxxx>
Sender: Al Viro <viro@xxxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Thu, Jan 12, 2012 at 02:20:53AM +0100, Jan Kara wrote:
> Replace racy xfs_wait_for_freeze() check with reliable sb_start_write()
> - sb_end_write() locking. Due to lock ranking dictated by the page fault
> code we have to call sb_start_write() after we acquire ilock.
> 
> CC: Ben Myers <bpm@xxxxxxx>
> CC: Alex Elder <elder@xxxxxxxxxx>
> Signed-off-by: Jan Kara <jack@xxxxxxx>
> ---
>  fs/xfs/xfs_file.c |    6 ++++--
>  1 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 753ed9b..c5f879b 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -862,9 +862,11 @@ xfs_file_dio_aio_write(
>               *iolock = XFS_IOLOCK_SHARED;
>       }
>  
> +     sb_start_write(inode->i_sb);
>       trace_xfs_file_direct_write(ip, count, iocb->ki_pos, 0);
>       ret = generic_file_direct_write(iocb, iovp,
>                       &nr_segs, pos, &iocb->ki_pos, count, ocount);
> +     sb_end_write(inode->i_sb);

What lock ordering do you have in mind?  Explicit description in fs/super.c,
please...

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