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


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