On Mon Dec 15, 2025 at 10:06 AM UTC, Yeoreum Yun wrote:
[snip]

quoted

Overall I am feeling a bit uncomfortable about this use of _nolock, but
I am also feeling pretty ignorant about PREEMPT_RT and also about this
arm64 code, so I am hesitant to suggest alternatives, I hope someone
else can offer some input here...

I understand. However, as I mentioned earlier,
my main intention was to hear opinions specifically about memory contention.

That said, if there is no memory contention,
I don’t think using the _nolock API is necessarily a bad approach.

In fact, I believe a bigger issue is that, under PREEMPT_RT,
code that uses the regular memory allocation APIs may give users the false impression
that those APIs are “safe to use,” even though they are not.

Yeah, I share this concern. I would bet I have written code that's
broken under PREEMPT_RT (luckily only in Google's kernel fork). The
comment for GFP_ATOMIC says:

 * %GFP_ATOMIC users can not sleep and need the allocation to succeed. A lower
 * watermark is applied to allow access to "atomic reserves".
 * The current implementation doesn't support NMI and few other strict
 * non-preemptive contexts (e.g. raw_spin_lock). The same applies to %GFP_NOWAIT.

It kinda sounds like it's supposed to be OK to use GFP_ATOMIC in a
normal preempt_disable() context. So do you know exactly why it's
invalid to use it in this stop_machine() context here? Maybe we need to
update this comment. Or, maybe actually we need to fix the allocator so
that GFP_ATOMIC allocs are safe in this context?