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

[Yao Zi]

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 at 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 at vger.kernel.org
> > Signed-off-by: Levi Zim <rsworktech at 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 at 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/