Re: [PATCH 1/5] locking/qspinlock: Safely handle > 4 nesting levels

[Waiman Long <longman@redhat.com>]

On 01/21/2019 04:12 AM, Peter Zijlstra wrote:
> On Sun, Jan 20, 2019 at 09:49:50PM -0500, Waiman Long wrote:
>> +/**
>> + *  acquire_lock_no_node - acquire lock without MCS node
>> + *  @lock: Pointer to queued spinlock structure
>> + *
>> + *  It is extremely unlikely that this function will ever be called.
>> + *  Marking it as noinline to not mess with the slowpath code. This
>> + *  function is for native qspinlock only. The PV qspinlock code has
>> + *  its own simpler version.
>> + *
>> + *  -----          -----  |   ----      -----          -----
>> + * |Tail2| <- ... |Head2| |  |Node| <- |Tail1| <- ... |Head1|
>> + *  -----          -----  |   ----      -----          -----
>> + *                   |                                   |
>> + *                   V                                   V
>> + *             Spin on waiting                     Spin on locked
>> + *
>> + * The waiting and the pending bits will be acquired first which are now
>> + * used as a separator for the disjointed queue shown above.
>> + *
>> + * The current CPU will then be inserted into queue by placing a special
>> + * _Q_TAIL_WAITING value into the tail and makes the current tail
>> + * point to its own local node. The next incoming CPU will see the special
>> + * tail, but it has no way to find the node. Instead, it will spin on the
>> + * waiting bit. When that bit is cleared, it means that all the the
>> + * previous CPUs in the queue are gone and current CPU is the new lock
>> + * holder. 
> I know it's monday morning and I've not had wake-up juice yet, but I
> don't think that's true.
>
> Consider there being two CPUs that ran out of nodes and thus we have two
> tail fragments waiting on the one waiting bit.

The waiting bit acts like a bit lock as no more than one can have it at
any time. The loser just keep spinning on it.

> There is no sane wait to recover from this.. and stay fair, why are we
> trying?
>
> That is; what's the problem with the below?
>
> Yes it sucks, but it is simple and doesn't introduce 100+ lines of code
> that 'never' gets used.
>
> ---
> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> index 8a8c3c208c5e..983b49a75826 100644
> --- a/kernel/locking/qspinlock.c
> +++ b/kernel/locking/qspinlock.c
> @@ -412,6 +412,12 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
>  	idx = node->count++;
>  	tail = encode_tail(smp_processor_id(), idx);
>  
> +	if (idx >= MAX_NODES) {
> +		while (!queued_spin_trylock(lock))
> +			cpu_relax();
> +		goto release;
> +	}
> +
>  	node = grab_mcs_node(node, idx);
>  
>  	/*

Yes, that can work too. Although there is a possibility of live lock, it
should seldom happen when we are talking about NMIs.

Cheers,
Longman