Re: [PATCH v4 14/16] locking/rwsem: Guard against making count negative

[Waiman Long]

On 04/18/2019 09:51 AM, Peter Zijlstra wrote:
> On Sat, Apr 13, 2019 at 01:22:57PM -0400, Waiman Long wrote:
>>  inline void __down_read(struct rw_semaphore *sem)
>>  {
>> +	long count = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS,
>> +						   &sem->count);
>> +
>> +	if (unlikely(count & RWSEM_READ_FAILED_MASK)) {
>> +		rwsem_down_read_failed(sem, count);
>>  		DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
>>  	} else {
>>  		rwsem_set_reader_owned(sem);
> *groan*, that is not provably correct. It is entirely possible to get
> enough fetch_add()s piled on top of one another to overflow regardless.
>
> Unlikely, yes, impossible, no.
>
> This makes me nervious as heck, I really don't want to ever have to
> debug something like that :-(

The number of fetch_add() that can pile up is limited by the number of
CPUs available in the system. Yes, if you have a 32k processor system
that have all the CPUs trying to acquire the same read-lock, we will
have a problem. Or as Linus had said that if we could have tasks kept
preempted right after doing the fetch_add with newly scheduled tasks
doing the fetch_add at the same lock again, we could have overflow with
less CPUs. How about disabling preemption before fetch_all and re-enable
it afterward to address the latter concern? I have no solution for the
first case, though.

Cheers,
Longman