CVS commit: src

["Taylor R Campbell" <riastradh%netbsd.org@localhost>]

Module Name:    src
Committed By:   riastradh
Date:           Sat Feb 12 17:10:02 UTC 2022

Modified Files:
        src/common/lib/libc/arch/mips/atomic: membar_ops.S
        src/sys/arch/mips/include: lock.h

Log Message:
mips: Brush up __cpu_simple_lock.

- Eradicate last vestiges of mb_* barriers.

- In __cpu_simple_lock_init, omit needless barrier.  It is the
  caller's responsibility to ensure __cpu_simple_lock_init happens
  before other operations on it anyway, so there was never any need
  for a barrier here.

- In __cpu_simple_lock_try, leave comments about memory ordering
  guarantees of the kernel's _atomic_cas_uint, which are inexplicably
  different from the non-underscored atomic_cas_uint.

- In __cpu_simple_unlock, use membar_exit instead of mb_memory, and do
  it unconditionally.

  This ensures that in __cpu_simple_lock/.../__cpu_simple_unlock, all
  memory operations in the ellipsis happen before the store that
  releases the lock.

  - On Octeon, the barrier was omitted altogether, which is a bug --
    it needs to be there or else there is no happens-before relation
    and whoever takes the lock next might see stale values stored or
    even stomp over the unlocking CPU's delayed loads.

  - On non-Octeon, the mb_memory was sync.  Using membar_exit
    preserves this.

  XXX On Octeon, membar_exit only issues syncw -- this seems wrong,
  only store-before-store and not load/store-before-store, unless the
  CNMIPS architecture guarantees it is sufficient here like
  SPARCv8/v9 PSO (`Partial Store Order').

- Leave an essay with citations about why we have an apparently
  pointless syncw _after_ releasing a lock, to work around a design
  bug^W^Wquirk in cnmips which sometimes buffers stores for hundreds
  of thousands of cycles for fun unless you issue syncw.


To generate a diff of this commit:
cvs rdiff -u -r1.10 -r1.11 src/common/lib/libc/arch/mips/atomic/membar_ops.S
cvs rdiff -u -r1.21 -r1.22 src/sys/arch/mips/include/lock.h

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.