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

[Vlastimil Babka (SUSE)]

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...

>> 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.