verity: use atomic_set instead of atomic_cmpxchg in read_completed

verity: use atomic_set instead of atomic_cmpxchg in read_completed

When moving from the PENDING to READY state it is sufficient to use
an atomic_set since we know the previous state MUST have been
PENDING. In addition, there is a BUG_ON which verifies this.

BUG=9752
TEST=Ran tests in verity.git. Ran platform_DMVerityCorruption on H/W.

Signed-off-by: Mandeep Singh Baines <msb@chromium.org>;
kernel.git Review URL: http://codereview.chromium.org/6677048

TBRing. Already LGTMed and committed to kernel.git

Change-Id: I1a199fc8f27f315bd98baa98f5fca0bb0fb4a556

R=wad@chromium.org,taysom@chromium.org,ups@chromium.org

Committed: http://chrome-svn/viewvc/chromeos?view=rev&revision=f782127

[Paul T, 2011-03-23 15:00:15]

LGTM - Though I'm still wondering why you can assume that another thread can't
set that flag.

[Mandeep Singh Baines, 2011-03-23 20:09:39]

On 2011/03/23 15:00:15, Paul T wrote:
> LGTM - Though I'm still wondering why you can assume that another thread can't
> set that flag.

The state machine guarantees it. The state transition to PENDING results in the
bio being submitted. We don't look at the bio or touch the entry again until the
completion. At which point, we transition from PENDING to READY.