linux-kernel - Re: [PATCH RFC cmpxchg 3/8] ARC: Emulate one-byte and two-byte cmpxchg

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Date: Tue, 02 Apr 2024 10:14:08 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Paul E. McKenney" <paulmck@...nel.org>, linux-kernel@...r.kernel.org
Cc: kernel-team@...a.com, "Vineet Gupta" <vgupta@...nel.org>,
 "Andi Shyti" <andi.shyti@...ux.intel.com>,
 "Andrzej Hajda" <andrzej.hajda@...el.com>,
 "Palmer Dabbelt" <palmer@...osinc.com>, linux-snps-arc@...ts.infradead.org
Subject: Re: [PATCH RFC cmpxchg 3/8] ARC: Emulate one-byte and two-byte cmpxchg

On Mon, Apr 1, 2024, at 23:39, Paul E. McKenney wrote:
> Use the new cmpxchg_emu_u8() and cmpxchg_emu_u16() to emulate one-byte
> and two-byte cmpxchg() on arc.
>
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>

I'm missing the context here, is it now mandatory to have 16-bit
cmpxchg() everywhere? I think we've historically tried hard to
keep this out of common code since it's expensive on architectures
that don't have native 16-bit load/store instructions (alpha, armv3)
and or sub-word atomics (armv5, riscv, mips).

Does the code that uses this rely on working concurrently with
non-atomic stores to part of the 32-bit word? If we want to
allow that, we need to merge my alpha ev4/45/5 removal series
first.

For the cmpxchg() interface, I would prefer to handle the
8-bit and 16-bit versions the same way as cmpxchg64() and
provide separate cmpxchg8()/cmpxchg16()/cmpxchg32() functions
by architectures that operate on fixed-size integer values
but not compounds or pointers, and a generic cmpxchg() wrapper
in common code that can handle the abtraction for pointers,
long and (if absolutely necessary) compounds by multiplexing
between cmpxchg32() and cmpxchg64() where needed.

I did a prototype a few years ago and found that there is
probably under a dozen users of the sub-word atomics in
the tree, so this mostly requires changes to architecture
code and less to drivers and core code.

      Arnd