Re: [PATCH RFC v3 04/10] bpf: Add lock-free cell for NMI-safe async operations

[Mykyta Yatsenko <mykyta.yatsenko5@gmail.com>]

On 1/7/26 18:30, Kumar Kartikeya Dwivedi wrote:
> On Wed, 7 Jan 2026 at 18:49, Mykyta Yatsenko <mykyta.yatsenko5@gmail.com> wrote:
>> From: Mykyta Yatsenko <yatsenko@meta.com>
>>
>> Introduce mpmc_cell, a lock-free cell primitive designed to support
>> concurrent writes to struct in NMI context (only one writer advances),
>> allowing readers to consume consistent snapshot.
>>
>> Implementation details:
>>   Double buffering allows writers run concurrently with readers (read
>>   from one cell, write to another)
>>
>>   The implementation uses a sequence-number-based protocol to enable
>>   exclusive writes.
>>    * Bit 0 of seq indicates an active writer
>>    * Bits 1+ form a generation counter
>>    * (seq & 2) >> 1 selects the read cell, write cell is opposite
>>    * Writers atomically set bit 0, write to the inactive cell, then
>>      increment seq to publish
>>    * Readers snapshot seq, read from the active cell, then validate
>>      that seq hasn't changed
>>
>> mpmc_cell expects users to pre-allocate double buffers.
>>
>> Key properties:
>>   * Writers never block (fail if lost the race to another writer)
>>   * Readers never block writers (double buffering), but may require
>>   retries if write updates the snapshot concurrently.
>>
>> This will be used by BPF timer and workqueue helpers to defer NMI-unsafe
>> operations (like hrtimer_start()) to irq_work effectively allowing BPF
>> programs to initiate timers and workqueues from NMI context.
>>
>> Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
>> ---
> We already have a dual-versioned concurrency control primitive in the
> kernel (seqcount_latch_t). I would just use that instead of
> reinventing it here. I don't see much of a difference except writer
> serialization, which can be done on top of it. If it was already
> considered and discarded for some reason, please add that reason to
> the commit message.
yes, multiple concurrent  writers support would is the main difference
between seqcount_latch_t and my implementation. I'll switch to
seqcount_latch_t and external synchronization for writers.
>>   [...]
>>