Re: [PATCH 3/6] selftests/bpf: libarena: Disable IRQs during allocation

["Kumar Kartikeya Dwivedi" <memxor@gmail.com>]

On Mon Aug 17, 2026 at 9:28 PM CEST, sashiko-bot wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
>
> Pre-existing issues:
> - [High] Unhandled lock failure path in `buddy_alloc` and `buddy_alloc_from_new_chunk` leaks newly allocated memory chunks.
> --
>
> --- Patch [3]: [PATCH 3/6] selftests/bpf: libarena: Disable IRQs during allocation ---
> commit 33cf3ed979bbaa281367dbd5d393dc12a91a1af9
> Author: Emil Tsalapatis <emil@etsalapatis.com>
>
> selftests/bpf: libarena: Disable IRQs during allocation
>
> This commit updates the buddy allocator in libarena to use IRQ-saving
> spinlocks to prevent ABBA deadlocks. These deadlocks were observed with
> sched_ext schedulers when an IRQ interrupts an allocation and subsequently
> attempts to take runqueue locks.
>
>> diff --git a/tools/testing/selftests/bpf/libarena/src/buddy.bpf.c b/tools/testing/selftests/bpf/libarena/src/buddy.bpf.c
>> index c674ee5cfcc1d..2490ab1396de0 100644
>> --- a/tools/testing/selftests/bpf/libarena/src/buddy.bpf.c
>> +++ b/tools/testing/selftests/bpf/libarena/src/buddy.bpf.c
>
> [ ... ]
>
>> @@ -730,9 +727,10 @@ static u64 buddy_alloc_from_existing_chunks(struct buddy __arena *buddy, int ord
>>   */
>>  static u64 buddy_alloc_from_new_chunk(struct buddy __arena *buddy, struct buddy_chunk __arena *chunk, int order)
>>  {
>> +	unsigned long flags;
>>  	u64 address;
>>
>> -	if (buddy_lock(buddy))
>> +	if (buddy_lock(buddy, flags))
>>  		return (u64)NULL;
>
> [Severity: High]
> This isn't a bug introduced by this patch, but does this leak the newly
> allocated arena chunk if the lock acquisition fails?
>
> When buddy_alloc() needs more memory, it allocates a chunk and passes it
> here:
>
> buddy_alloc()
>   chunk = buddy_chunk_get(buddy);
>   if (chunk)
>     address = buddy_alloc_from_new_chunk(buddy, chunk, order);
>
> If buddy_lock() fails in buddy_alloc_from_new_chunk() (for example, returning
> an error like -ETIMEDOUT under heavy contention), it returns NULL without
> linking the new chunk to buddy->first_chunk.
>
> The NULL return then propagates up to buddy_alloc(), which discards the chunk
> pointer completely without freeing it. Since BPF MCS spinlocks can fail under
> heavy task load, could this predictably exhaust arena memory?
>
> Should this error path call bpf_arena_free_pages() on the orphaned chunk
> before returning?
>

The rest are meh, but this one seems plausible. Please check and follow up.

>>
>>
>> @@ -745,7 +743,7 @@ static u64 buddy_alloc_from_new_chunk(struct buddy __arena *buddy, struct buddy_
>>
>>  	address = buddy_chunk_alloc(buddy->first_chunk, order);
>>
>> -	buddy_unlock(buddy);
>> +	buddy_unlock(buddy, flags);
>>
>>  	return (u64)address;
>>  }
>
> [ ... ]