Re: [PATCH 3/5] xfs: DIO write completion size updates race

Brian Foster
2015-04-10T20:22:12Z

On Fri, Apr 10, 2015 at 11:37:58PM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> xfs_end_io_direct_write() can race with other IO completions when
> updating the in-core inode size. The IO completion processing is not
> serialised for direct IO - they are done either under the
> IOLOCK_SHARED for non-AIO DIO, and without any IOLOCK held at all
> during AIO DIO completion. Hence the non-atomic test-and-set update
> of the in-core inode size is racy and can result in the in-core
> inode size going backwards if the race if hit just right.
>
> If the inod size goes backwards, this can trigger the EOF zeroing
> code to run incorrectly on the next IO, which then will zero data
> that has successfully been written to disk by a previous DIO.
>
> To fix this bug, we need to serialise the test/set updates of the
> in-core inode size. This first patch introduces locking around the
> relevant updates and checks in the DIO path. Because we now have an
> ioend in xfs_end_io_direct_write(), we know exactly then we are
> doing an IO that requires an in-core EOF update, and we know that
> they are not running in interrupt context. As such, we do not need to
> use irqsave() spinlock variants to protect against interrupts while
> the lock is held.
>
> Hence we can use an existing spinlock in the inode to do this
> serialisation and so not need to grow the struct xfs_inode just to
> work around this problem.
>
> This patch does not address the test/set EOF update in
> generic_file_write_direct() for various reasons - that will be done
> as a followup with separate explanation.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
> fs/xfs/xfs_aops.c | 17 ++++++++++++-----
> fs/xfs/xfs_file.c | 13 ++++++++++++-
> 2 files changed, 24 insertions(+), 6 deletions(-)
>
> diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
> index 52c7e46..aafd54c 100644
> --- a/fs/xfs/xfs_aops.c
> +++ b/fs/xfs/xfs_aops.c
> @@ -1616,21 +1616,28 @@ xfs_end_io_direct_write(
> /*
> * The ioend tells us whether we are doing unwritten extent conversion
> * or an append transaction that updates the on-disk file size. These
> - * cases are the only cases where we should *potentially* be needing
> - * to update the VFS inode size. When the ioend indicates this, we
> - * are *guaranteed* to be running in non-interrupt context.
> + * cases are the only cases where we should *potentially* be needing to
> + * update the VFS inode size. When the ioend indicates this, we are
> + * *guaranteed* to be running in non-interrupt context.
> *
> * We need to update the in-core inode size here so that we don't end up
> * with the on-disk inode size being outside the in-core inode size.
> * While we can do this in the process context after the IO has
> - * completed, this does not work for AIO and hence we always update
> - * the in-core inode size here if necessary.
> + * completed, this does not work for AIO and hence we always update the
> + * in-core inode size here if necessary.
> + *
> + * We need to lock the test/set EOF update as we can be racing with
> + * other IO completions here to update the EOF. Failing to serialise
> + * here can result in EOF moving backwards and Bad Things Happen when
> + * that occurs.
> */
> + spin_lock(&ip->i_flags_lock);
> if (ioend->io_type == XFS_IO_UNWRITTEN || ioend->io_append_trans) {
> if (offset + size > i_size_read(inode))
> i_size_write(inode, offset + size);
> } else
> ASSERT(offset + size <= i_size_read(inode));
> + spin_unlock(&ip->i_flags_lock);

Looks good to me once we fix the (known) locking problem above of taking
the spinlock before checking the ioend (e.g., having a lock cycle in irq
context):

Reviewed-by: Brian Foster <bfoster@redhat.com>

>
> /* Ugh. No way to propagate errors, so ignore them. */
> if (ioend->io_type == XFS_IO_UNWRITTEN) {
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index dc5f609..38ff356 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -569,10 +569,20 @@ restart:
> * write. If zeroing is needed and we are currently holding the
> * iolock shared, we need to update it to exclusive which implies
> * having to redo all checks before.
> + *
> + * We need to serialise against EOF updates that occur in IO
> + * completions here. We want to make sure that nobody is changing the
> + * size while we do this check until we have placed an IO barrier (i.e.
> + * hold the XFS_IOLOCK_EXCL) that prevents new IO from being dispatched.
> + * The spinlock effectively forms a memory barrier once we have the
> + * XFS_IOLOCK_EXCL so we are guaranteed to see the latest EOF value
> + * and hence be able to correctly determine if we need to run zeroing.
> */
> + spin_lock(&ip->i_flags_lock);
> if (*pos > i_size_read(inode)) {
> bool zero = false;
>
> + spin_unlock(&ip->i_flags_lock);
> if (*iolock == XFS_IOLOCK_SHARED) {
> xfs_rw_iunlock(ip, *iolock);
> *iolock = XFS_IOLOCK_EXCL;
> @@ -582,7 +592,8 @@ restart:
> error = xfs_zero_eof(ip, *pos, i_size_read(inode), &zero);
> if (error)
> return error;
> - }
> + } else
> + spin_unlock(&ip->i_flags_lock);
>
> /*
> * Updating the timestamps will grab the ilock again from
> --
> 2.0.0
>
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