On Mon, 22 Nov 2010 15:22:08 -0800
"Darrick J. Wong" <djwong@us.ibm.com> wrote:

quoted text
> Before 2.6.37, the md layer had a mechanism for catching I/Os with the barrier
> flag set, and translating the barrier into barriers for all the underlying
> devices.  With 2.6.37, I/O barriers have become plain old flushes, and the md
> code was updated to reflect this.  However, one piece was left out -- the md
> layer does not tell the block layer that it supports flushes or FUA access at
> all, which results in md silently dropping flush requests.
> 
> Since the support already seems there, just add this one piece of bookkeeping
> to restore the ability to flush writes through md.

I would rather just unconditionally call
   blk_queue_flush(mddev->queue, REQ_FLUSH | REQ_FUA);

I don't think there is much to be gained by trying to track exactly what the
underlying devices support, and as the devices can change, that is racy
anyway.

Thoughts?

NeilBrown




quoted text
> 
> Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
> ---
> 
>  drivers/md/md.c |   25 ++++++++++++++++++++++++-
>  1 files changed, 24 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index 324a366..a52d7be 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -356,6 +356,21 @@ EXPORT_SYMBOL(mddev_congested);
>  /*
>   * Generic flush handling for md
>   */
> +static void evaluate_flush_capability(mddev_t *mddev)
> +{
> +	mdk_rdev_t *rdev;
> +	unsigned int flush = REQ_FLUSH | REQ_FUA;
> +
> +	rcu_read_lock();
> +	list_for_each_entry_rcu(rdev, &mddev->disks, same_set) {
> +		if (rdev->raid_disk < 0)
> +			continue;
> +		flush &= rdev->bdev->bd_disk->queue->flush_flags;
> +	}
> +	rcu_read_unlock();
> +
> +	blk_queue_flush(mddev->queue, flush);
> +}
>  
>  static void md_end_flush(struct bio *bio, int err)
>  {
> @@ -1885,6 +1900,8 @@ static int bind_rdev_to_array(mdk_rdev_t * rdev, mddev_t * mddev)
>  	/* May as well allow recovery to be retried once */
>  	mddev->recovery_disabled = 0;
>  
> +	evaluate_flush_capability(mddev);
> +
>  	return 0;
>  
>   fail:
> @@ -1903,17 +1920,23 @@ static void md_delayed_delete(struct work_struct *ws)
>  static void unbind_rdev_from_array(mdk_rdev_t * rdev)
>  {
>  	char b[BDEVNAME_SIZE];
> +	mddev_t *mddev;
> +
>  	if (!rdev->mddev) {
>  		MD_BUG();
>  		return;
>  	}
> -	bd_release_from_disk(rdev->bdev, rdev->mddev->gendisk);
> +	mddev = rdev->mddev;
> +	bd_release_from_disk(rdev->bdev, mddev->gendisk);
>  	list_del_rcu(&rdev->same_set);
>  	printk(KERN_INFO "md: unbind<%s>\n", bdevname(rdev->bdev,b));
>  	rdev->mddev = NULL;
>  	sysfs_remove_link(&rdev->kobj, "block");
>  	sysfs_put(rdev->sysfs_state);
>  	rdev->sysfs_state = NULL;
> +
> +	evaluate_flush_capability(mddev);
> +
>  	/* We need to delay this, otherwise we can deadlock when
>  	 * writing to 'remove' to "dev/state".  We also need
>  	 * to delay it due to rcu usage.

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