Fix use of WITNESS_UNLOCK() in rw_exit_read() and rw_exit_write().

WITNESS_UNLOCK() has to be called before the actual lock is released.
Otherwise, the checker would trigger a use-after-free if the rwlock was
dynamically allocated and another thread freed it too early.

In addition to fixing the lock release issue, this patch does
the following improvements:

* membar_exit_before_atomic() is now invoked only once per lock release.
* rwl_owner is read as late as possible to make rw_cas() failure
  less likely.
* The rw_cas() of rw_exit() (now rw_do_exit()) is put inside
  __predict_false(). This compacts the resulting machine code a bit.

Tested by and OK anton@
OK mpi@

0 files changed, 0 insertions, 0 deletions