Re: [PATCH bpf v3] bpf: do not use kmalloc_nolock when !HAVE_CMPXCHG_DOUBLE

From: Yao Zi <me@ziyao.cc>
To: Paul Chaignon <paul.chaignon@gmail.com>, rsworktech@outlook.com
Cc: Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Martin KaFai Lau <martin.lau@linux.dev>,
	Eduard Zingerman <eddyz87@gmail.com>, Song Liu <song@kernel.org>,
	Yonghong Song <yonghong.song@linux.dev>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>,
	Stanislav Fomichev <sdf@fomichev.me>, Hao Luo <haoluo@google.com>,
	Jiri Olsa <jolsa@kernel.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Clark Williams <clrkwllms@kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Paul Walmsley <pjw@kernel.org>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Alexandre Ghiti <alex@ghiti.fr>, Amery Hung <ameryhung@gmail.com>,
	linux-riscv@lists.infradead.org, stable@vger.kernel.org,
	bpf@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-rt-devel@lists.linux.dev
Subject: Re: [PATCH bpf v3] bpf: do not use kmalloc_nolock when !HAVE_CMPXCHG_DOUBLE
Date: Tue, 17 Mar 2026 04:45:05 +0000	[thread overview]
Message-ID: <abjcUSXWiShiuMvH@pie> (raw)
In-Reply-To: <abgcKvuSQc2ZYKw4@mail.gmail.com>

On Mon, Mar 16, 2026 at 04:05:14PM +0100, Paul Chaignon wrote:
> On Sun, Mar 15, 2026 at 12:02:48AM +0800, Levi Zim via B4 Relay wrote:
> > From: Levi Zim <rsworktech@outlook.com>
> > 
> > kmalloc_nolock always fails for architectures that lack cmpxchg16b.
> > For example, this causes bpf_task_storage_get with flag
> > BPF_LOCAL_STORAGE_GET_F_CREATE to fails on riscv64 6.19 kernel.
> > 
> > Fix it by enabling use_kmalloc_nolock only when HAVE_CMPXCHG_DOUBLE.
> > But leave the PREEMPT_RT case as is because it requires kmalloc_nolock
> > for correctness. Add a comment about this limitation that architecture's
> > lack of CMPXCHG_DOUBLE combined with PREEMPT_RT could make
> > bpf_local_storage_alloc always fail.
> > 
> > Fixes: f484f4a3e058 ("bpf: Replace bpf memory allocator with kmalloc_nolock() in local storage")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Levi Zim <rsworktech@outlook.com>
> > ---
> 
> Note there may be something broken with your setup as lore is reporting
> that you sent this v3 email three times. Not sure if it could be an
> issue.

Once is because linux-riscv@lists.infradead.org adds a trailer when
forwarding messages but keeps the Message-ID unchanged, so lore indexed
one extra message with the same ID but different content, it was not
Levi doing something wrong.

The other message has the same content but a different From line, not
sure what happened to it. Differences of the messages could be viewed
here[1].

Regards,
Yao Zi

[1]: https://lore.kernel.org/all/20260315-bpf-kmalloc-nolock-v3-1-91c72bf91902@outlook.com/d/