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

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Message-ID: <Z8ZOFuipuznnOD-7@google.com>
Date: Tue, 4 Mar 2025 00:49:26 +0000
From: Peilin Ye <yepeilin@...gle.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.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 03, 2025 at 04:45:45PM -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.

Got it.

> In core.c we probably cannot, but here we can.
> So yes. please respin.

Sure!

> I bet the compiler will produce the exact same code.

Thanks,
Peilin Ye