linux-kernel - Re: [PATCH v2] LoongArch/percpu: Simplify _percpu_read() and _percpu

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Message-ID: <CAFULd4abR8Upu5UecVes-sEc9his2yWL7pg8hwKSJ+ORSNL5Ag@mail.gmail.com>
Date: Wed, 4 Sep 2024 17:24:13 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: Huacai Chen <chenhuacai@...nel.org>
Cc: loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org, 
	WANG Xuerui <kernel@...0n.name>, Xi Ruoyao <xry111@...111.site>, 
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2] LoongArch/percpu: Simplify _percpu_read() and _percpu_write()

On Wed, Sep 4, 2024 at 5:02 PM Huacai Chen <chenhuacai@...nel.org> wrote:
>
> Hi, Uros,
>
> Thank you for your patch.
>
> On Wed, Sep 4, 2024 at 5:52 PM Uros Bizjak <ubizjak@...il.com> wrote:
> >
> > _percpu_read() and _percpu_write() macros call __percpu_read()
> > and __percpu_write() static inline functions that result in a single
> > assembly instruction. Percpu infrastructure expects its leaf
> > definitions to encode the size of their percpu variable, so the patch
> > merges asm clauses from the static inline function into the
> > corresponding leaf macros.
> It seems in some other places we prefer inline functions rather than
> macros, but this patch is the opposite...

Please note that these are leaf macros (functions), always used
through the upper level macro (see e.g. the definition of
raw_cpu_read() and __pcpu_size_call_return() in
include/linux/percpu-defs.h). These upper level macros do type check
on the pointer, so there is no need to do it again in the leaf macro.
The percpu address space checks on x86 depend on the presence of these
checks.

> >
> > The secondary effect of this change is to avoid explicit __percpu
> > function arguments. Currently, __percpu macro is defined in
> > include/linux/compiler_types.h, but with proposed patch [1],
> > __percpu definition will need macros from include/asm-generic/percpu.h,
> > creating forward dependency loop.
> Macros don't check types, so use macros to drop "__percpu" checking?
> Seems a little strange.

As explained above, types are checked in the upper level macro (that
uses these leaf macros) through __verify_pcpu_ptr(). These checks
currently use sparse to check __percpu tag, but x86 will soon use the
compiler infrastructure with much more powerful checks in this place.

So, there is really no need to type check percpu pointer also in leaf functions.

Best regards,
Uros.