[PATCH 0/1] sparc32: prevent stale writes by failed bit-lock waiters

[Magnus Lindholm <linmag7@gmail.com>]

I traced an SMP HyperSPARC pathname-lookup livelock to a failed bit-lock
waiter restoring an unlinked dentry. Skip unchanged stores in the SPARC32
emulated atomics and bit helpers, without changing generic bitops or ABI.

For background, PA-RISC made test-and-set/clear conditional in 2006
(af5917f0cd60). The 2018 generic rewrite (84c6591103db) routed bit locks
through atomic_fetch_*(), which needs the same unchanged-store rule.
I keep this fix within arch/sparc; PA-RISC's fetch helpers need separate
review and are outside this patch's scope. The SPARC32 fetch helpers,
including fetch-add, gain a comparison; I claim no performance improvement.

The forced-interleaving host test reproduces the old lost update and
checks the corrected generic acquisition path. SMP and UP builds and
QEMU smoke tests passed for the architecture-only implementation.

I completed a native GDB 18.1 build on a dual-CPU HyperSPARC RT626
SPARCstation 20 with 512 MB RAM and a native SPARC32 SMP kernel containing
the combined fixes and optimizations, plus separate platform prerequisites.
I observed no core dumps, livelock or new kernel messages during the run.
This validates the combined integration, not each patch in isolation.

Magnus Lindholm (1):
  sparc32: do not write back unchanged values in emulated atomics

 arch/sparc/lib/atomic32.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)


base-commit: cee9395acd8043be0644b25c34bfa86623f2b935
-- 
2.43.0