[PATCH v2] mm/slub: Genericize x86-specific cmpxchg16b comment

[PATCH v2] mm/slub: Genericize x86-specific cmpxchg16b comment

[Tiezhu Yang]

The kmalloc_nolock() infrastructure relies on slab caches with double-word
atomic cmpxchg capabilities (__CMPXCHG_DOUBLE). The current inline comment
in __kmalloc_nolock_noprof() uses 'cmpxchg16b' as the prerequisite hardware
capability.

However, "cmpxchg16b" is an x86-specific instruction mnemonic (CX16). On
the modern architectures supported by the Linux kernel, this double-width
atomic primitive is satisfied by different hardware implementations:
(1) arm64: satisfies it via LSE atomics ('casp' instruction)
(2) s390: satisfies it via native 'cdsg' instruction
(3) riscv: satisfies it via ZACAS extension ('amocas.q' instruction)
(4) loongarch: satisfies it via SCQ extension (using an 'll.d'/'sc.q' loop)

Update the comment to "double-width atomic cmpxchg", which provides the
following advantages:
(1) Accurately describes the cross-platform hardware invariant.
(2) Covers both 32-bit and 64-bit platforms.
(3) Aligns well with the __CMPXCHG_DOUBLE flag.
(4) Avoids misleading developers of non-x86 architectures.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
---
v2:
  - Update "128-bit atomic cmpxchg" to "double-width atomic cmpxchg" to
    properly cover both 32-bit and 64-bit platforms, suggested by Harry.
  - Refine the commit message to format the architecture implementations
    and advantages into structured lists for better readability.

 mm/slub.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 45e077c7526c..090302f1daeb 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -5589,8 +5589,8 @@ static void *__kmalloc_nolock_noprof(DECL_TOKEN_PARAMS(size, token), gfp_t gfp_f
 	if (!(s->flags & __CMPXCHG_DOUBLE) && !kmem_cache_debug(s))
 		/*
 		 * kmalloc_nolock() is not supported on architectures that
-		 * don't implement cmpxchg16b and thus need slab_lock()
-		 * which could be preempted by a nmi.
+		 * don't implement double-width atomic cmpxchg and thus need
+		 * slab_lock() which could be preempted by a nmi.
 		 * But debug caches don't use that and only rely on
 		 * kmem_cache_node->list_lock, so kmalloc_nolock() can attempt
 		 * to allocate from debug caches by
-- 
2.42.0

Re: [PATCH v2] mm/slub: Genericize x86-specific cmpxchg16b comment

[Alexei Starovoitov]

On Sun, Nov 08, 2026 at 07:59 PM Tiezhu Yang <yangtiezhu@loongson.cn> wrote:
> atomic primitive is satisfied by different hardware implementations:
> (1) arm64: satisfies it via LSE atomics ('casp' instruction)
[...]
> Update the comment to "double-width atomic cmpxchg", which provides the
> following advantages:
> (1) Accurately describes the cross-platform hardware invariant.

You ignored Hao's point about shorter commit logs.

I dropped all of your pending patches.

pw-bot: cr