Re: [PATCH] locking/qspinlock: Add bug check for exceeding MAX_NODES

[Waiman Long <longman@redhat.com>]

On 01/16/2019 11:47 AM, Will Deacon wrote:
> On Tue, Jan 15, 2019 at 04:55:44PM -0500, Waiman Long wrote:
>> On some architectures, it is possible to have nested NMIs taking
>> spinlocks nestedly. Even though the chance of having more than 4 nested
>> spinlocks with contention is extremely small, there could still be a
>> possibility that it may happen some days leading to system panic.
>>
>> What we don't want is a silent corruption with system panic somewhere
>> else. So add a BUG_ON() check to make sure that a system panic caused
>> by this will show the correct root cause.
>>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>>  kernel/locking/qspinlock.c | 10 ++++++++++
>>  1 file changed, 10 insertions(+)
>>
>> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
>> index 8a8c3c2..f823221 100644
>> --- a/kernel/locking/qspinlock.c
>> +++ b/kernel/locking/qspinlock.c
>> @@ -412,6 +412,16 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
>>  	idx = node->count++;
>>  	tail = encode_tail(smp_processor_id(), idx);
>>  
>> +	/*
>> +	 * 4 nodes are allocated based on the assumption that there will
>> +	 * not be nested NMIs taking spinlocks. That may not be true in
>> +	 * some architectures even though the chance of needing more than
>> +	 * 4 nodes will still be extremely unlikely. Adding a bug check
>> +	 * here to make sure there won't be a silent corruption in case
>> +	 * this condition happens.
>> +	 */
>> +	BUG_ON(idx >= MAX_NODES);
>> +
> Hmm, I really don't like the idea of putting a BUG_ON() on the spin_lock()
> path. I'd prefer it if (a) we didn't add extra conditional code for the
> common case and (b) didn't bring down the machine. Could we emit a
> lockdep-style splat, instead?
>
> Will

I am going to drop this patch. I am working on another one that will
handle the no MCS node available case by spinning directly on the lock
cacheline under this rare circumstance. Of course, that will incur a
little bit of performance overhead in the slow path which I am trying to
measure right now.

Cheers,
Longman