Are you using dm-crypt? Then this may be related to another bug that appeared on 4.0. See:
http://thread.gmane.org/gmane.linux.kernel/1942014 The following issue on RH's tracker is also related: https://bugzilla.redhat.com/show_bug.cgi?id=1223332 I can confirm that last bug (dm-crypt). I have experimented this issue after upgrading to 4.0.2 (lot of libata errors). Luckily I quickly noticed it and downgraded to 3.16, and I didn't suffered of any data corruption/loss (or at least didn't noticed so far). Regards.
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