linux-kernel - [PATCH v5 2/4] atomic: Specify alignment for atomic_t and atomic64

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Message-ID: <d6cd17b387fa4b4cf9f419ec586ac4756bc7aaeb.1765866665.git.fthain@linux-m68k.org>
Date: Tue, 16 Dec 2025 17:31:05 +1100
From: Finn Thain <fthain@...ux-m68k.org>
To: Peter Zijlstra <peterz@...radead.org>,
    Will Deacon <will@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
    Arnd Bergmann <arnd@...db.de>,
    Boqun Feng <boqun.feng@...il.com>,
    Gary Guo <gary@...yguo.net>,
    Mark Rutland <mark.rutland@....com>,
    linux-arch@...r.kernel.org,
    linux-kernel@...r.kernel.org,
    linux-m68k@...ts.linux-m68k.org,
    Guo Ren <guoren@...nel.org>,
    linux-csky@...r.kernel.org,
    Geert Uytterhoeven <geert@...ux-m68k.org>,
    Dinh Nguyen <dinguyen@...nel.org>,
    Jonas Bonn <jonas@...thpole.se>,
    Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>,
    Stafford Horne <shorne@...il.com>,
    linux-openrisc@...r.kernel.org,
    Yoshinori Sato <ysato@...rs.sourceforge.jp>,
    Rich Felker <dalias@...c.org>,
    John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
    linux-sh@...r.kernel.org
Subject: [PATCH v5 2/4] atomic: Specify alignment for atomic_t and atomic64_t

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@...nel.org>
Cc: linux-csky@...r.kernel.org
Cc: Geert Uytterhoeven <geert@...ux-m68k.org>
Cc: linux-m68k@...ts.linux-m68k.org
Cc: Dinh Nguyen <dinguyen@...nel.org>
Cc: Jonas Bonn <jonas@...thpole.se>
Cc: Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>
Cc: Stafford Horne <shorne@...il.com>
Cc: linux-openrisc@...r.kernel.org
Cc: Yoshinori Sato <ysato@...rs.sourceforge.jp>
Cc: Rich Felker <dalias@...c.org>
Cc: John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>
Cc: linux-sh@...r.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@...ux-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