Device loses barrier support (was: Fixed patch for simple barriers.)

[Andi Kleen]

> If you are pushing what you are pushing --- barriers allowing to return 
> EOPNOTSUPP anytime --- then asynchronous barrier submits can no longer be 
> used, because by the time EOPNOTSUPP is detected, the filesystem is 
> already corrupted.

Chris Mason pointed out that this can actually already happen. From
a quick review this can happen in MD raid1 at least (their barriers_work
flag is pretty similar to the DM implementation I did).  So everyone
has to handle this already anyways.

> I'm wondering, where in fsync() does Linux wait for hardware disk cache to 
> be flushed? Isn't there a bug that fsync() will return before the cache is 
> flushed? I couldn't really find it. The last thing do_fsync calls is 
> filemap_fdatawait and it doesn't do cache flush (blkdev_issue_flush).

At least in fsync() on journaling fs the metadata update should push it.

-Andi

-- 
ak at linux.intel.com