Re: [bpf-next v5 3/3] bpf: hash map, suppress false lockdep warning

[Martin KaFai Lau <martin.lau@linux.dev>]

On 1/13/23 1:15 AM, Tonghao Zhang wrote:
> 
> 
>> On Jan 13, 2023, at 9:53 AM, Martin KaFai Lau <martin.lau@linux.dev> wrote:
>>
>> On 1/11/23 1:29 AM, tong@infragraf.org wrote:
>>> +	/*
>>> +	 * The lock may be taken in both NMI and non-NMI contexts.
>>> +	 * There is a false lockdep warning (inconsistent lock state),
>>> +	 * if lockdep enabled. The potential deadlock happens when the
>>> +	 * lock is contended from the same cpu. map_locked rejects
>>> +	 * concurrent access to the same bucket from the same CPU.
>>> +	 * When the lock is contended from a remote cpu, we would
>>> +	 * like the remote cpu to spin and wait, instead of giving
>>> +	 * up immediately. As this gives better throughput. So replacing
>>> +	 * the current raw_spin_lock_irqsave() with trylock sacrifices
>>> +	 * this performance gain. atomic map_locked is necessary.
>>> +	 * lockdep_off is invoked temporarily to fix the false warning.
>>> +	 */
>>> +	lockdep_off();
>>>   	raw_spin_lock_irqsave(&b->raw_lock, flags);
>>> -	*pflags = flags;
>>> +	lockdep_on();
>>
>> I am not very sure about the lockdep_off/on. Other than the false warning when using the very same htab map by both NMI and non-NMI context, I think the lockdep will still be useful to catch other potential issues. The commit c50eb518e262 ("bpf: Use separate lockdep class for each hashtab") has already solved this false alarm when NMI happens on one map and non-NMI happens on another map.
>>
>> Alexei, what do you think? May be only land the patch 1 fix for now.
> Hi Martin
> Patch 2 is used for patch 1 to test whether there is a deadlock. We should apply this two patches.

It is too noisy for test_progs that developers routinely run. Lets continue to 
explore other ways (or a different test) without this false positive splat. 
Patch 1 was applied as already mentioned in the earlier reply.