[PATCH v5 0/4] Align atomic storage

[PATCH v5 0/4] Align atomic storage

[Finn Thain]

This series adds the __aligned attribute to atomic_t and atomic64_t
definitions in include/linux and include/asm-generic (respectively)
to get natural alignment of both types on csky, m68k, microblaze,
nios2, openrisc and sh.

This series also adds Kconfig options to enable a new run-time warning
to help reveal misaligned atomic accesses on platforms which don't
trap that.

The performance impact is expected to vary across platforms and workloads.
The measurements I made on m68k show that some workloads run faster and
others slower.

---

Changed since v4:
 - Dropped parisc header file patch as it's been merged already.
 - Submitted as PATCH instead of RFC.

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 (3):
  bpf: Explicitly align bpf_res_spin_lock
  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

 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 ++++++++++++++++++
 6 files changed, 36 insertions(+), 4 deletions(-)

-- 
2.49.1

[PATCH v5 1/4] 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 becomes redundant.

Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: linux-m68k@lists.linux-m68k.org
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Finn Thain <fthain@linux-m68k.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

Re: [PATCH v5 1/4] bpf: Explicitly align bpf_res_spin_lock

[Arnd Bergmann]

On Tue, Dec 16, 2025, at 07:31, Finn Thain wrote:
> 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 becomes redundant.
>
> Cc: Geert Uytterhoeven <geert@linux-m68k.org>
> Cc: linux-m68k@lists.linux-m68k.org
> Acked-by: Alexei Starovoitov <ast@kernel.org>
> Signed-off-by: Finn Thain <fthain@linux-m68k.org>

Reviewed-by: Arnd Bergmann <arnd@arndb.de>

Re: [PATCH v5 0/4] Align atomic storage

[Andrew Morton]

On Tue, 16 Dec 2025 17:31:05 +1100 Finn Thain <fthain@linux-m68k.org> wrote:

> This series adds the __aligned attribute to atomic_t and atomic64_t
> definitions in include/linux and include/asm-generic (respectively)
> to get natural alignment of both types on csky, m68k, microblaze,
> nios2, openrisc and sh.
> 
> This series also adds Kconfig options to enable a new run-time warning
> to help reveal misaligned atomic accesses on platforms which don't
> trap that.
> 
> The performance impact is expected to vary across platforms and workloads.
> The measurements I made on m68k show that some workloads run faster and
> others slower.

Looks nice, thanks.

I don't know which tree this is aimed at.  I grabbed a copy for
mm.git's mm-nonmm-unstable tree, can drop it again if this pops up in
linux-next via another route.