locking-core-2020-10-12

 - Add deadlock detection for recursive read-locks. The rationale is outlined
   in:

     224ec489d3cd: ("lockdep/Documention: Recursive read lock detection reasoning")

   The main deadlock pattern we want to detect is:

           TASK A:                 TASK B:

           read_lock(X);
                                   write_lock(X);
           read_lock_2(X);

 - Add "latch sequence counters" (seqcount_latch_t):

      A sequence counter variant where the counter even/odd value is used to
      switch between two copies of protected data. This allows the read path,
      typically NMIs, to safely interrupt the write side critical section.

   We utilize this new variant for sched-clock, and to make x86 TSC handling safer.

 - Other seqlock cleanups, fixes and enhancements

 - KCSAN updates

 - LKMM updates

 - Misc updates, cleanups and fixes.

Signed-off-by: Ingo Molnar <mingo@kernel.org>