linux-kernel - Re: [PATCH bpf-next v5 1/6] bpf: Introduce load-acquire and store-release instructions

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Message-ID: <CAADnVQKB-9q6fxcVPbd7Ee+QBH=_ySv2EyULkgFhv_n2i07L1A@mail.gmail.com>
Date: Mon, 3 Mar 2025 16:45:45 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Peilin Ye <yepeilin@...gle.com>
Cc: bpf <bpf@...r.kernel.org>, 
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>, bpf@...f.org, 
	Alexei Starovoitov <ast@...nel.org>, Xu Kuohai <xukuohai@...weicloud.com>, 
	Eduard Zingerman <eddyz87@...il.com>, David Vernet <void@...ifault.com>, 
	Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>, Jonathan Corbet <corbet@....net>, 
	"Paul E. McKenney" <paulmck@...nel.org>, Puranjay Mohan <puranjay@...nel.org>, 
	Ilya Leoshkevich <iii@...ux.ibm.com>, Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>, 
	Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, 
	Quentin Monnet <qmo@...nel.org>, Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>, 
	Ihor Solodrai <ihor.solodrai@...ux.dev>, Yingchi Long <longyingchi24s@....ac.cn>, 
	Josh Don <joshdon@...gle.com>, Barret Rhoden <brho@...gle.com>, Neel Natu <neelnatu@...gle.com>, 
	Benjamin Segall <bsegall@...gle.com>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v5 1/6] bpf: Introduce load-acquire and
 store-release instructions

On Mon, Mar 3, 2025 at 4:39 PM Peilin Ye <yepeilin@...gle.com> wrote:
>
> Hi Alexei,
>
> On Mon, Mar 03, 2025 at 04:24:12PM -0800, Alexei Starovoitov wrote:
> > >         switch (insn->imm) {
> > > @@ -7780,6 +7813,24 @@ static int check_atomic(struct bpf_verifier_env *env, struct bpf_insn *insn)
> > >         case BPF_XCHG:
> > >         case BPF_CMPXCHG:
> > >                 return check_atomic_rmw(env, insn);
> > > +       case BPF_LOAD_ACQ:
> > > +#ifndef CONFIG_64BIT
> > > +               if (BPF_SIZE(insn->code) == BPF_DW) {
> > > +                       verbose(env,
> > > +                               "64-bit load-acquires are only supported on 64-bit arches\n");
> > > +                       return -EOPNOTSUPP;
> > > +               }
> > > +#endif
> >
> > Your earlier proposal of:
> > if (BPF_SIZE(insn->code) == BPF_DW && BITS_PER_LONG != 64) {
> >
> > was cleaner.
> > Why did you pick ifndef ?
>
> Likely overthinking, but I wanted to avoid this check at all for 64-bit
> arches, so it's just a little bit faster.  Should I change it back to
> checking BITS_PER_LONG ?

In general #ifdef in .c is the last resort.
We avoid it when possible.
In core.c we probably cannot, but here we can.
So yes. please respin.
I bet the compiler will produce the exact same code.