linux-kernel - [RFC v3 3/5] atomic: Specify alignment for atomic_t and atomic64

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Message-ID: <d5a0121eb3c9dc3645488d24a2f71aa55582e75f.1759875560.git.fthain@linux-m68k.org>
Date: Wed, 08 Oct 2025 09:19:20 +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>,
    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 v3 3/5] 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.

On an m68k system with 14 MB of RAM, this patch reduces the available
memory by a couple of percent. On a 64 MB system, the cost is under 1%
but still significant. I don't know whether there is sufficient
performance gain to justify the memory cost; it still has to be measured.

Cc: Lance Yang <lance.yang@...ux.dev>
Link: https://lore.kernel.org/lkml/CAMuHMdW7Ab13DdGs2acMQcix5ObJK0O2dG_Fxzr8_g58Rc1_0g@mail.gmail.com/
---
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