[RFC v4 0/5] Align atomic storage

[RFC v4 0/5] Align atomic storage

[Finn Thain]

This series adds the __aligned attribute to atomic_t and atomic64_t
definitions in include/asm-generic.

It also adds Kconfig options to enable a new runtime warning to help
reveal misaligned atomic accesses on platforms which don't trap that.

This patch series is a Request For Comments because the alignment
change is a time/space tradeoff. Its costs and benefits are expected
to vary across platforms and workloads. More measurements are needed.

---

Changed since v3:
 - Rebased on v6.17.
 - New patch to resolve header dependency issue on parisc.
 - Dropped documentation patch.

Changed since v2:
 - Specify natural alignment for atomic64_t.
 - CONFIG_DEBUG_ATOMIC checks for natural alignment again.
 - New patch to add weakened alignment check.
 - New patch for explicit alignment in BPF header.

---

Finn Thain (4):
  bpf: Explicitly align bpf_res_spin_lock
  parisc: Drop linux/kernel.h include from asm/bug.h header
  atomic: Specify alignment for atomic_t and atomic64_t
  atomic: Add option for weaker alignment check

Peter Zijlstra (1):
  atomic: Add alignment check to instrumented atomic operations

 arch/parisc/include/asm/bug.h    |  2 --
 include/asm-generic/atomic64.h   |  2 +-
 include/asm-generic/rqspinlock.h |  2 +-
 include/linux/instrumented.h     | 15 +++++++++++++++
 include/linux/types.h            |  2 +-
 kernel/bpf/rqspinlock.c          |  1 -
 lib/Kconfig.debug                | 18 ++++++++++++++++++
 7 files changed, 36 insertions(+), 6 deletions(-)

-- 
2.49.1

[RFC v4 1/5] bpf: Explicitly align bpf_res_spin_lock

[Finn Thain]

Align bpf_res_spin_lock to avoid a BUILD_BUG_ON() when the alignment
changes, as it will do on m68k when, in a subsequent patch, the minimum
alignment of the atomic_t member of struct rqspinlock gets increased
from 2 to 4. Drop the BUILD_BUG_ON() as it is now redundant.

Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Alexei Starovoitov <ast@kernel.org>
---
 include/asm-generic/rqspinlock.h | 2 +-
 kernel/bpf/rqspinlock.c          | 1 -
 2 files changed, 1 insertion(+), 2 deletions(-)

diff --git a/include/asm-generic/rqspinlock.h b/include/asm-generic/rqspinlock.h
index 6d4244d643df..eac2dcd31b83 100644
--- a/include/asm-generic/rqspinlock.h
+++ b/include/asm-generic/rqspinlock.h
@@ -28,7 +28,7 @@ struct rqspinlock {
  */
 struct bpf_res_spin_lock {
 	u32 val;
-};
+} __aligned(__alignof__(struct rqspinlock));
 
 struct qspinlock;
 #ifdef CONFIG_QUEUED_SPINLOCKS
diff --git a/kernel/bpf/rqspinlock.c b/kernel/bpf/rqspinlock.c
index a00561b1d3e5..02f1f671e624 100644
--- a/kernel/bpf/rqspinlock.c
+++ b/kernel/bpf/rqspinlock.c
@@ -692,7 +692,6 @@ __bpf_kfunc int bpf_res_spin_lock(struct bpf_res_spin_lock *lock)
 	int ret;
 
 	BUILD_BUG_ON(sizeof(rqspinlock_t) != sizeof(struct bpf_res_spin_lock));
-	BUILD_BUG_ON(__alignof__(rqspinlock_t) != __alignof__(struct bpf_res_spin_lock));
 
 	preempt_disable();
 	ret = res_spin_lock((rqspinlock_t *)lock);
-- 
2.49.1