[PATCH v6 0/4] Align atomic storage

[PATCH v6 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 v5:
 - Added acked-by and revewed-by tags.
 - Added a new macro to inhibit emission of __bug_table section, for the
 benefit of pre-boot code like the EFI stub loader.

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

 arch/x86/boot/compressed/Makefile     |  1 +
 drivers/firmware/efi/libstub/Makefile |  1 +
 include/asm-generic/atomic64.h        |  2 +-
 include/asm-generic/rqspinlock.h      |  2 +-
 include/linux/instrumented.h          | 17 +++++++++++++++++
 include/linux/types.h                 |  2 +-
 kernel/bpf/rqspinlock.c               |  1 -
 lib/Kconfig.debug                     | 18 ++++++++++++++++++
 8 files changed, 40 insertions(+), 4 deletions(-)

-- 
2.49.1

[PATCH v6 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>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Finn Thain <fthain@linux-m68k.org>
---

Changed since v5:
 - Added tag from Arnd Bergmann.
---
 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 0f2dcbbfee2f..dd36ac96bf66 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 f7d0c8d4644e..8d892fb099ac 100644
--- a/kernel/bpf/rqspinlock.c
+++ b/kernel/bpf/rqspinlock.c
@@ -694,7 +694,6 @@ __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

[PATCH v6 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/
Acked-by: Guo Ren <guoren@kernel.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Finn Thain <fthain@linux-m68k.org>
---
Changed since v5:
 - Added tags from Guo Ren and Arnd Bergmann.

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 d4437e9c452c..1760e1feeab9 100644
--- a/include/linux/types.h
+++ b/include/linux/types.h
@@ -180,7 +180,7 @@ 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

[PATCH v6 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.

[fthain: added __DISABLE_BUG_TABLE macro.]

Cc: Sasha Levin <sashal@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: x86@kernel.org
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: https://lore.kernel.org/lkml/20250901093600.GF4067720@noisy.programming.kicks-ass.net/
Link: https://lore.kernel.org/linux-next/df9fbd22-a648-ada4-fee0-68fe4325ff82@linux-m68k.org/
Signed-off-by: Finn Thain <fthain@linux-m68k.org>
---
Checkpatch.pl says...
ERROR: Missing Signed-off-by: line by nominal patch author 'Peter Ziljstra <peterz@infradead.org>'
---
Changed since v5:
 - Add new __DISABLE_BUG_TABLE macro to prevent a build failure on those
architectures which use atomics in pre-boot code like the EFI stub loader:

x86_64-linux-gnu-ld: error: unplaced orphan section `__bug_table' from `arch/x86/boot/compressed/sev-handle-vc.o'

Changed since v2:
 - Always check for natural alignment.
---
 arch/x86/boot/compressed/Makefile     |  1 +
 drivers/firmware/efi/libstub/Makefile |  1 +
 include/linux/instrumented.h          | 10 ++++++++++
 lib/Kconfig.debug                     | 10 ++++++++++
 4 files changed, 22 insertions(+)

diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
index 68f9d7a1683b..122967c80e48 100644
--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -42,6 +42,7 @@ KBUILD_CFLAGS += -Wno-microsoft-anon-tag
 endif
 KBUILD_CFLAGS += -Wno-pointer-sign
 KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
+KBUILD_CFLAGS += -D__DISABLE_BUG_TABLE
 KBUILD_CFLAGS += -D__DISABLE_EXPORTS
 # Disable relocation relaxation in case the link is not PIE.
 KBUILD_CFLAGS += $(call cc-option,-Wa$(comma)-mrelax-relocations=no)
diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
index 7d15a85d579f..ac3e7c64aedb 100644
--- a/drivers/firmware/efi/libstub/Makefile
+++ b/drivers/firmware/efi/libstub/Makefile
@@ -42,6 +42,7 @@ KBUILD_CFLAGS			:= $(subst $(CC_FLAGS_FTRACE),,$(cflags-y)) \
 				   -ffreestanding \
 				   -fno-stack-protector \
 				   $(call cc-option,-fno-addrsig) \
+				   -D__DISABLE_BUG_TABLE \
 				   -D__DISABLE_EXPORTS
 
 #
diff --git a/include/linux/instrumented.h b/include/linux/instrumented.h
index 711a1f0d1a73..bcd1113b55a1 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 +68,9 @@ static __always_inline void instrument_atomic_read(const volatile void *v, size_
 {
 	kasan_check_read(v, size);
 	kcsan_check_atomic_read(v, size);
+#ifndef __DISABLE_BUG_TABLE
+	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (size - 1)));
+#endif
 }
 
 /**
@@ -81,6 +85,9 @@ static __always_inline void instrument_atomic_write(const volatile void *v, size
 {
 	kasan_check_write(v, size);
 	kcsan_check_atomic_write(v, size);
+#ifndef __DISABLE_BUG_TABLE
+	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (size - 1)));
+#endif
 }
 
 /**
@@ -95,6 +102,9 @@ static __always_inline void instrument_atomic_read_write(const volatile void *v,
 {
 	kasan_check_write(v, size);
 	kcsan_check_atomic_read_write(v, size);
+#ifndef __DISABLE_BUG_TABLE
+	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (size - 1)));
+#endif
 }
 
 /**
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index ba36939fda79..4b4d1445ef9c 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1359,6 +1359,16 @@ 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 v6 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 v5:
 - Rebased.

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 | 25 ++++++++++++++++---------
 lib/Kconfig.debug            |  8 ++++++++
 2 files changed, 24 insertions(+), 9 deletions(-)

diff --git a/include/linux/instrumented.h b/include/linux/instrumented.h
index bcd1113b55a1..e8983eb8a888 100644
--- a/include/linux/instrumented.h
+++ b/include/linux/instrumented.h
@@ -56,6 +56,19 @@ 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)
+{
+#ifndef __DISABLE_BUG_TABLE
+	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);
+	}
+#endif
+}
+
 /**
  * instrument_atomic_read - instrument atomic read access
  * @v: address of access
@@ -68,9 +81,7 @@ static __always_inline void instrument_atomic_read(const volatile void *v, size_
 {
 	kasan_check_read(v, size);
 	kcsan_check_atomic_read(v, size);
-#ifndef __DISABLE_BUG_TABLE
-	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (size - 1)));
-#endif
+	instrument_atomic_check_alignment(v, size);
 }
 
 /**
@@ -85,9 +96,7 @@ static __always_inline void instrument_atomic_write(const volatile void *v, size
 {
 	kasan_check_write(v, size);
 	kcsan_check_atomic_write(v, size);
-#ifndef __DISABLE_BUG_TABLE
-	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (size - 1)));
-#endif
+	instrument_atomic_check_alignment(v, size);
 }
 
 /**
@@ -102,9 +111,7 @@ static __always_inline void instrument_atomic_read_write(const volatile void *v,
 {
 	kasan_check_write(v, size);
 	kcsan_check_atomic_read_write(v, size);
-#ifndef __DISABLE_BUG_TABLE
-	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (size - 1)));
-#endif
+	instrument_atomic_check_alignment(v, size);
 }
 
 /**
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 4b4d1445ef9c..977a2c9163a2 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1369,6 +1369,14 @@ 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

Re: [PATCH v6 3/4] atomic: Add alignment check to instrumented atomic operations

[Peter Zijlstra]

On Wed, Dec 31, 2025 at 07:25:42PM +1100, Finn Thain wrote:
> 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.
> 
> [fthain: added __DISABLE_BUG_TABLE macro.]
> 
> Cc: Sasha Levin <sashal@kernel.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: x86@kernel.org
> Cc: Ard Biesheuvel <ardb@kernel.org>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Link: https://lore.kernel.org/lkml/20250901093600.GF4067720@noisy.programming.kicks-ass.net/
> Link: https://lore.kernel.org/linux-next/df9fbd22-a648-ada4-fee0-68fe4325ff82@linux-m68k.org/
> Signed-off-by: Finn Thain <fthain@linux-m68k.org>
> ---
> Checkpatch.pl says...
> ERROR: Missing Signed-off-by: line by nominal patch author 'Peter Ziljstra <peterz@infradead.org>'
> ---
> Changed since v5:
>  - Add new __DISABLE_BUG_TABLE macro to prevent a build failure on those
> architectures which use atomics in pre-boot code like the EFI stub loader:
> 
> x86_64-linux-gnu-ld: error: unplaced orphan section `__bug_table' from `arch/x86/boot/compressed/sev-handle-vc.o'

Urgh, so why not simply use __DISABLE_EXPORTS, that's typically (ab)used
for these things?

Also, unless __DISABLE_BUG_TABLE goes live inside asm/bug.h and kills
all __bug_table emissions, its a misnomer.

Furthermore, that SEV thing is broken and needs to be fixed anyway, this
isn't helping it much. noinstr code should not be using instrumented
things to begin with.

Re: [PATCH v6 3/4] atomic: Add alignment check to instrumented atomic operations

[Finn Thain]

On Mon, 5 Jan 2026, Peter Zijlstra wrote:

> On Wed, Dec 31, 2025 at 07:25:42PM +1100, Finn Thain wrote:
> > 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.
> > 
> > [fthain: added __DISABLE_BUG_TABLE macro.]
> > 
> > Cc: Sasha Levin <sashal@kernel.org>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: Borislav Petkov <bp@alien8.de>
> > Cc: Dave Hansen <dave.hansen@linux.intel.com>
> > Cc: x86@kernel.org
> > Cc: Ard Biesheuvel <ardb@kernel.org>
> > Cc: "H. Peter Anvin" <hpa@zytor.com>
> > Link: https://lore.kernel.org/lkml/20250901093600.GF4067720@noisy.programming.kicks-ass.net/
> > Link: https://lore.kernel.org/linux-next/df9fbd22-a648-ada4-fee0-68fe4325ff82@linux-m68k.org/
> > Signed-off-by: Finn Thain <fthain@linux-m68k.org>
> > ---
> > Checkpatch.pl says...
> > ERROR: Missing Signed-off-by: line by nominal patch author 'Peter Ziljstra <peterz@infradead.org>'
> > ---
> > Changed since v5:
> >  - Add new __DISABLE_BUG_TABLE macro to prevent a build failure on those
> > architectures which use atomics in pre-boot code like the EFI stub loader:
> > 
> > x86_64-linux-gnu-ld: error: unplaced orphan section `__bug_table' from `arch/x86/boot/compressed/sev-handle-vc.o'
> 
> Urgh, so why not simply use __DISABLE_EXPORTS, that's typically (ab)used
> for these things?
> 

OK, I'll change it back to __DISABLE_EXPORTS.

> Also, unless __DISABLE_BUG_TABLE goes live inside asm/bug.h and kills
> all __bug_table emissions, its a misnomer.
> 

Yes, __DISABLE_BUG_TABLE is certainly a misnomer, since what it actually 
does is to elide code that would emit bug table entries. I would argue 
that this distinction is splitting hairs but it's moot.

> Furthermore, that SEV thing is broken and needs to be fixed anyway, this 
> isn't helping it much. noinstr code should not be using instrumented 
> things to begin with.
> 

The problem is not confined to x86. I needed something that would work for 
loongarch, arm, riscv etc.

Anyway, thanks for your review. I will make the necessary changes.