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

[Peter Zijlstra]

On Thu, Apr 18, 2019 at 10:08:28AM -0400, Waiman Long wrote:
> 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.

Having more CPUs than that is not impossible these days.

> 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.

That.

> How about disabling preemption before fetch_all and re-enable
> it afterward to address the latter concern? 

Performance might be an issue, look at what preempt_disable() +
preempt_enable() generate for ARM64 for example. That's not particularly
pretty.

> I have no solution for the first case, though.

A cmpxchg() loop can fix this, but that again has performance
implications like you mentioned a while back.