Re: [RFC PATCH] mm: restrict can_spin_trylock() to preemptible context on PREEMPT_RT

[Harry Yoo]

On Fri, Sep 25, 2026 at 09:29:16AM +0200, Vlastimil Babka (SUSE) wrote:
> On 9/25/26 06:12, Alexei Starovoitov wrote:
> > On Tue, Sep 22, 2026 at 08:04 AM Karl Mehltretter <kmehltretter@xxxxxxxxx> wrote:
> >> a) Change MM so _nolock() allocation remains safe and can still succeed
> >>    while pi_lock or an rq lock is held. Existing BPF behavior would
> >>    remain unchanged. Is this feasible, or would it require substantial
> >>    allocator changes?
> > 
> > I don't think it's feasible. On RT the sheaves lock, n->list_lock and
> > zone->lock are all rtmutex based. When trylock succeeded and a waiter
> > showed up rt_spin_unlock() has to wake it.
> > 
> >> b) Restore BPF local storage's dedicated allocator, and document, with
> >>    debug checks if possible, that _nolock() must not be used in those
> >>    scheduler-lock contexts.
> > 
> > No. Local storage is not the only one. bpf_arena_alloc_pages(),
> > bpf_stream_vprintk(), bpf_task_work_schedule_*() call kmalloc_nolock()
> > as well and the same sched_waking prog can call them.
> > bpf_mem_alloc was removed from local storage, because it wastes memory
> > on preallocation. Same answer for d).
> > 
> >> c) Have MM detect those contexts and return NULL. This prevents the
> >>    deadlock while preserving ordinary raw-lock callers such as arena,
> >>    but task-storage creation from scheduler tracepoints then fails
> >>    deterministically.
> > 
> > This one.
> 
> Yeah, but how exactly to do that detection :)
> 
> > bpf_task_storage_get() with F_CREATE can return NULL and the progs
> > have to check for it anyway.
> 
> e) Introduce raw_local_trylock_t...

+1 on this :)

> >> Are these the right alternatives? In particular, what behavior is
> >> intended from the _nolock() API in these scheduler-lock contexts?
> > 
> > It can be called from any context and it can return NULL.
> > That's what it does from NMI and hardirq on RT already.

-- 
Cheers,
Harry / Hyeonggon