Messages in this thread | | | Subject | Re: [PATCH v4 14/16] locking/rwsem: Guard against making count negative | From | Waiman Long <> | Date | Thu, 18 Apr 2019 10:54:19 -0400 |
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On 04/18/2019 10:40 AM, Peter Zijlstra wrote: > 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. >
Having more than 32k CPUs contending for the same cacheline will be horribly slow.
>> 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.
That is just for the preempt kernel. Right? Thinking about it some more, the above scenario is less likely to happen for CONFIG_PREEMPT_VOLUNTARY kernel and the preempt_disable cost will be lower. A preempt RT kernel is less likely to run on system with many CPUs anyway. We could make that a conifg option as well in a follow-on patch and let the distributors decide.
>> 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.
Exactly.
Cheers, Longman
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