On Fri, Aug 20, 2021 at 09:24:58AM +0100, Will Deacon wrote:

quoted

quoted

gcc-10.2.1, x86_64-defconfig

kernel/event/core.o-inline-ud1:     96454
kernel/event/core.o-outofline-ud1:  96604
kernel/event/core.o-outofline-call: 97072

    kernel/event/core.o-outofline-saturate-ud2: 96954
    kernel/event/core.o:                97248

quoted

Is that with the saturation moved to the UD handler as well? 

Yep, that's the full function call replaced with an exception.

quoted

I think it would be good to keep that as close to the point at which
we detect the problem as we can, so perhaps we can inline that part
and leave the diagnostics to the exception handler?

That's simpler execption code too, we can abuse the existing WARN/UD2
stuff.

---
 arch/x86/include/asm/refcount.h | 31 +++++++++++++++++++++++++++++++
 include/asm-generic/bug.h       |  4 ++++
 include/linux/refcount.h        | 15 +++++++++++----
 lib/bug.c                       | 13 ++++++++++++-
 lib/refcount.c                  |  7 ++-----
 5 files changed, 60 insertions(+), 10 deletions(-)

diff --git a/arch/x86/include/asm/refcount.h b/arch/x86/include/asm/refcount.h
new file mode 100644
index 000000000000..bed52b95d24c
--- /dev/null
+++ b/arch/x86/include/asm/refcount.h

@@ -0,0 +1,31 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_REFCOUNT_H
+#define _ASM_X86_REFCOUNT_H
+
+#define refcount_warn_saturate refcount_warn_saturate
+static __always_inline void refcount_warn_saturate(refcount_t *r, const enum refcount_saturation_type t)
+{
+	refcount_set(r, REFCOUNT_SATURATED);
+	__WARN_FLAGS(BUGFLAG_ONCE|BUGFLAG_REFCOUNT|BUGFLAG_REFCOUNT_TYPE(t));
+}