[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 4/4] atomic: Add option for weaker alignment check

[Finn Thain]

Add a new Kconfig symbol to make CONFIG_DEBUG_ATOMIC more useful on
those architectures which do not align dynamic allocations to 8-byte
boundaries. Without this, CONFIG_DEBUG_ATOMIC produces excessive
WARN splats.

Signed-off-by: Finn Thain <fthain@linux-m68k.org>
---
Changed since v3:
 - Dropped #include <linux/cache.h> to avoid a build failure on arm64.
 - Rewrote Kconfig help text to better describe preferred alignment.
 - Refactored to avoid line-wrapping and duplication.
---
 include/linux/instrumented.h | 17 ++++++++++++++---
 lib/Kconfig.debug            |  8 ++++++++
 2 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/include/linux/instrumented.h b/include/linux/instrumented.h
index 402a999a0d6b..ca946c5860be 100644
--- a/include/linux/instrumented.h
+++ b/include/linux/instrumented.h
@@ -56,6 +56,17 @@ static __always_inline void instrument_read_write(const volatile void *v, size_t
 	kcsan_check_read_write(v, size);
 }
 
+static __always_inline void instrument_atomic_check_alignment(const volatile void *v, size_t size)
+{
+	if (IS_ENABLED(CONFIG_DEBUG_ATOMIC)) {
+		unsigned int mask = size - 1;
+
+		if (IS_ENABLED(CONFIG_DEBUG_ATOMIC_LARGEST_ALIGN))
+			mask &= sizeof(struct { long x; } __aligned_largest) - 1;
+		WARN_ON_ONCE((unsigned long)v & mask);
+	}
+}
+
 /**
  * instrument_atomic_read - instrument atomic read access
  * @v: address of access
@@ -68,7 +79,7 @@ static __always_inline void instrument_atomic_read(const volatile void *v, size_
 {
 	kasan_check_read(v, size);
 	kcsan_check_atomic_read(v, size);
-	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (size - 1)));
+	instrument_atomic_check_alignment(v, size);
 }
 
 /**
@@ -83,7 +94,7 @@ static __always_inline void instrument_atomic_write(const volatile void *v, size
 {
 	kasan_check_write(v, size);
 	kcsan_check_atomic_write(v, size);
-	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (size - 1)));
+	instrument_atomic_check_alignment(v, size);
 }
 
 /**
@@ -98,7 +109,7 @@ static __always_inline void instrument_atomic_read_write(const volatile void *v,
 {
 	kasan_check_write(v, size);
 	kcsan_check_atomic_read_write(v, size);
-	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (size - 1)));
+	instrument_atomic_check_alignment(v, size);
 }
 
 /**
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index d080d23d9a87..60f9deef00fd 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1366,6 +1366,14 @@ config DEBUG_ATOMIC
 
 	  This option has potentially significant overhead.
 
+config DEBUG_ATOMIC_LARGEST_ALIGN
+	bool "Check alignment only up to __aligned_largest"
+	depends on DEBUG_ATOMIC
+	help
+	  If you say Y here then the check for natural alignment of
+	  atomic accesses will be constrained to the compiler's largest
+	  alignment for scalar types.
+
 menu "Lock Debugging (spinlocks, mutexes, etc...)"
 
 config LOCK_DEBUGGING_SUPPORT
-- 
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>

[PATCH v5 3/4] atomic: Add alignment check to instrumented atomic operations

[Finn Thain]

From: Peter Zijlstra <peterz@infradead.org>

Add a Kconfig option for debug builds which logs a warning when an
instrumented atomic operation takes place that's misaligned.
Some platforms don't trap for this.

Link: https://lore.kernel.org/lkml/20250901093600.GF4067720@noisy.programming.kicks-ass.net/
Signed-off-by: Finn Thain <fthain@linux-m68k.org>
---
Changed since v2:
 - Always check for natural alignment.
---
Checkpatch.pl says...
ERROR: Missing Signed-off-by: line by nominal patch author 'Peter Zijlstra <peterz@infradead.org>'
---
 include/linux/instrumented.h |  4 ++++
 lib/Kconfig.debug            | 10 ++++++++++
 2 files changed, 14 insertions(+)

diff --git a/include/linux/instrumented.h b/include/linux/instrumented.h
index 711a1f0d1a73..402a999a0d6b 100644
--- a/include/linux/instrumented.h
+++ b/include/linux/instrumented.h
@@ -7,6 +7,7 @@
 #ifndef _LINUX_INSTRUMENTED_H
 #define _LINUX_INSTRUMENTED_H
 
+#include <linux/bug.h>
 #include <linux/compiler.h>
 #include <linux/kasan-checks.h>
 #include <linux/kcsan-checks.h>
@@ -67,6 +68,7 @@ static __always_inline void instrument_atomic_read(const volatile void *v, size_
 {
 	kasan_check_read(v, size);
 	kcsan_check_atomic_read(v, size);
+	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (size - 1)));
 }
 
 /**
@@ -81,6 +83,7 @@ static __always_inline void instrument_atomic_write(const volatile void *v, size
 {
 	kasan_check_write(v, size);
 	kcsan_check_atomic_write(v, size);
+	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (size - 1)));
 }
 
 /**
@@ -95,6 +98,7 @@ static __always_inline void instrument_atomic_read_write(const volatile void *v,
 {
 	kasan_check_write(v, size);
 	kcsan_check_atomic_read_write(v, size);
+	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (size - 1)));
 }
 
 /**
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 713cc94caa02..d080d23d9a87 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1356,6 +1356,16 @@ config DEBUG_PREEMPT
 	  depending on workload as it triggers debugging routines for each
 	  this_cpu operation. It should only be used for debugging purposes.
 
+config DEBUG_ATOMIC
+	bool "Debug atomic variables"
+	depends on DEBUG_KERNEL
+	help
+	  If you say Y here then the kernel will add a runtime alignment check
+	  to atomic accesses. Useful for architectures that do not have trap on
+	  mis-aligned access.
+
+	  This option has potentially significant overhead.
+
 menu "Lock Debugging (spinlocks, mutexes, etc...)"
 
 config LOCK_DEBUGGING_SUPPORT
-- 
2.49.1

[PATCH v5 2/4] atomic: Specify alignment for atomic_t and atomic64_t

[Finn Thain]

Some recent commits incorrectly assumed 4-byte alignment of locks.
That assumption fails on Linux/m68k (and, interestingly, would have
failed on Linux/cris also). The jump label implementation makes a
similar alignment assumption.

The expectation that atomic_t and atomic64_t variables will be naturally
aligned seems reasonable, as indeed they are on 64-bit architectures.
But atomic64_t isn't naturally aligned on csky, m68k, microblaze, nios2,
openrisc and sh. Neither atomic_t nor atomic64_t are naturally aligned
on m68k.

This patch brings a little uniformity by specifying natural alignment
for atomic types. One benefit is that atomic64_t variables do not get
split across a page boundary. The cost is that some structs grow which
leads to cache misses and wasted memory.

Cc: Guo Ren <guoren@kernel.org>
Cc: linux-csky@vger.kernel.org
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: linux-m68k@lists.linux-m68k.org
Cc: Dinh Nguyen <dinguyen@kernel.org>
Cc: Jonas Bonn <jonas@southpole.se>
Cc: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
Cc: Stafford Horne <shorne@gmail.com>
Cc: linux-openrisc@vger.kernel.org
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: Rich Felker <dalias@libc.org>
Cc: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Cc: linux-sh@vger.kernel.org
Link: https://lore.kernel.org/lkml/CAFr9PX=MYUDGJS2kAvPMkkfvH+0-SwQB_kxE4ea0J_wZ_pk=7w@mail.gmail.com
Link: https://lore.kernel.org/lkml/CAMuHMdW7Ab13DdGs2acMQcix5ObJK0O2dG_Fxzr8_g58Rc1_0g@mail.gmail.com/
Signed-off-by: Finn Thain <fthain@linux-m68k.org>
---
Changed since v2:
 - Specify natural alignment for atomic64_t.
Changed since v1:
 - atomic64_t now gets an __aligned attribute too.
 - The 'Fixes' tag has been dropped because Lance sent a different fix
   for commit e711faaafbe5 ("hung_task: replace blocker_mutex with encoded
   blocker") that's suitable for -stable.
---
 include/asm-generic/atomic64.h | 2 +-
 include/linux/types.h          | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/asm-generic/atomic64.h b/include/asm-generic/atomic64.h
index 100d24b02e52..f22ccfc0df98 100644
--- a/include/asm-generic/atomic64.h
+++ b/include/asm-generic/atomic64.h
@@ -10,7 +10,7 @@
 #include <linux/types.h>
 
 typedef struct {
-	s64 counter;
+	s64 __aligned(sizeof(s64)) counter;
 } atomic64_t;
 
 #define ATOMIC64_INIT(i)	{ (i) }
diff --git a/include/linux/types.h b/include/linux/types.h
index 6dfdb8e8e4c3..a225a518c2c3 100644
--- a/include/linux/types.h
+++ b/include/linux/types.h
@@ -179,7 +179,7 @@ typedef phys_addr_t resource_size_t;
 typedef unsigned long irq_hw_number_t;
 
 typedef struct {
-	int counter;
+	int __aligned(sizeof(int)) counter;
 } atomic_t;
 
 #define ATOMIC_INIT(i) { (i) }
-- 
2.49.1

Re: [PATCH v5 2/4] atomic: Specify alignment for atomic_t and atomic64_t

[Guo Ren]

On Tue, Dec 16, 2025 at 2:38 PM Finn Thain <fthain@linux-m68k.org> wrote:
>
> Some recent commits incorrectly assumed 4-byte alignment of locks.
> That assumption fails on Linux/m68k (and, interestingly, would have
> failed on Linux/cris also). The jump label implementation makes a
> similar alignment assumption.
>
> The expectation that atomic_t and atomic64_t variables will be naturally
> aligned seems reasonable, as indeed they are on 64-bit architectures.
> But atomic64_t isn't naturally aligned on csky, m68k, microblaze, nios2,
> openrisc and sh. Neither atomic_t nor atomic64_t are naturally aligned
> on m68k.
>
> This patch brings a little uniformity by specifying natural alignment
> for atomic types. One benefit is that atomic64_t variables do not get
> split across a page boundary. The cost is that some structs grow which
> leads to cache misses and wasted memory.
>
> Cc: Guo Ren <guoren@kernel.org>
> Cc: linux-csky@vger.kernel.org
> Cc: Geert Uytterhoeven <geert@linux-m68k.org>
> Cc: linux-m68k@lists.linux-m68k.org
> Cc: Dinh Nguyen <dinguyen@kernel.org>
> Cc: Jonas Bonn <jonas@southpole.se>
> Cc: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
> Cc: Stafford Horne <shorne@gmail.com>
> Cc: linux-openrisc@vger.kernel.org
> Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
> Cc: Rich Felker <dalias@libc.org>
> Cc: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
> Cc: linux-sh@vger.kernel.org
> Link: https://lore.kernel.org/lkml/CAFr9PX=MYUDGJS2kAvPMkkfvH+0-SwQB_kxE4ea0J_wZ_pk=7w@mail.gmail.com
> Link: https://lore.kernel.org/lkml/CAMuHMdW7Ab13DdGs2acMQcix5ObJK0O2dG_Fxzr8_g58Rc1_0g@mail.gmail.com/
> Signed-off-by: Finn Thain <fthain@linux-m68k.org>
> ---
> Changed since v2:
>  - Specify natural alignment for atomic64_t.
> Changed since v1:
>  - atomic64_t now gets an __aligned attribute too.
>  - The 'Fixes' tag has been dropped because Lance sent a different fix
>    for commit e711faaafbe5 ("hung_task: replace blocker_mutex with encoded
>    blocker") that's suitable for -stable.
> ---
>  include/asm-generic/atomic64.h | 2 +-
>  include/linux/types.h          | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/include/asm-generic/atomic64.h b/include/asm-generic/atomic64.h
> index 100d24b02e52..f22ccfc0df98 100644
> --- a/include/asm-generic/atomic64.h
> +++ b/include/asm-generic/atomic64.h
> @@ -10,7 +10,7 @@
>  #include <linux/types.h>
>
>  typedef struct {
> -       s64 counter;
> +       s64 __aligned(sizeof(s64)) counter;
This alignment is okay for all.

Acked-by: Guo Ren <guoren@kernel.org>

>  } atomic64_t;
>
>  #define ATOMIC64_INIT(i)       { (i) }
> diff --git a/include/linux/types.h b/include/linux/types.h
> index 6dfdb8e8e4c3..a225a518c2c3 100644
> --- a/include/linux/types.h
> +++ b/include/linux/types.h
> @@ -179,7 +179,7 @@ typedef phys_addr_t resource_size_t;
>  typedef unsigned long irq_hw_number_t;
>
>  typedef struct {
> -       int counter;
> +       int __aligned(sizeof(int)) counter;
>  } atomic_t;
>
>  #define ATOMIC_INIT(i) { (i) }
> --
> 2.49.1
>


-- 
Best Regards
 Guo Ren

Re: [PATCH v5 2/4] atomic: Specify alignment for atomic_t and atomic64_t

[Arnd Bergmann]

On Tue, Dec 16, 2025, at 07:31, Finn Thain wrote:
> Some recent commits incorrectly assumed 4-byte alignment of locks.
> That assumption fails on Linux/m68k (and, interestingly, would have
> failed on Linux/cris also). The jump label implementation makes a
> similar alignment assumption.
>
> The expectation that atomic_t and atomic64_t variables will be naturally
> aligned seems reasonable, as indeed they are on 64-bit architectures.
> But atomic64_t isn't naturally aligned on csky, m68k, microblaze, nios2,
> openrisc and sh. Neither atomic_t nor atomic64_t are naturally aligned
> on m68k.
>
> This patch brings a little uniformity by specifying natural alignment
> for atomic types. One benefit is that atomic64_t variables do not get
> split across a page boundary. The cost is that some structs grow which
> leads to cache misses and wasted memory.

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.