linux-kernel - [RFC v2 2/3] atomic: Specify alignment for atomic_t and atomic64

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Message-ID: <abf2bf114abfc171294895b63cd00af475350dba.1757810729.git.fthain@linux-m68k.org>
Date: Sun, 14 Sep 2025 10:45:29 +1000
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>,
    Boqun Feng <boqun.feng@...il.com>,
    Jonathan Corbet <corbet@....net>,
    Mark Rutland <mark.rutland@....com>,
    Arnd Bergmann <arnd@...db.de>,
    linux-kernel@...r.kernel.org,
    linux-arch@...r.kernel.org,
    Geert Uytterhoeven <geert@...ux-m68k.org>,
    linux-m68k@...r.kernel.org,
    Lance Yang <lance.yang@...ux.dev>
Subject: [RFC v2 2/3] 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). Specify the minimum alignment of atomic
variables for fewer surprises and (hopefully) better performance.

Consistent with i386, atomic64_t is not given natural alignment here.

Cc: Lance Yang <lance.yang@...ux.dev>
Link: https://lore.kernel.org/lkml/CAMuHMdW7Ab13DdGs2acMQcix5ObJK0O2dG_Fxzr8_g58Rc1_0g@mail.gmail.com/
---
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..7ae82ac17645 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 counter __aligned(sizeof(long));
 } atomic64_t;
 
 #define ATOMIC64_INIT(i)	{ (i) }
diff --git a/include/linux/types.h b/include/linux/types.h
index 6dfdb8e8e4c3..cd5b2b0f4b02 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 counter __aligned(sizeof(int));
 } atomic_t;
 
 #define ATOMIC_INIT(i) { (i) }
-- 
2.49.1