Re: [PATCH RFC tip/core/rcu] SRCU rewrite

[Lai Jiangshan]

On Thu, Nov 17, 2016 at 10:31 PM, Boqun Feng <boqun.feng@xxxxxxxxx> wrote:
> On Thu, Nov 17, 2016 at 08:18:51PM +0800, Lai Jiangshan wrote:
>> On Tue, Nov 15, 2016 at 10:37 PM, Paul E. McKenney
>> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
>> > On Tue, Nov 15, 2016 at 09:44:45AM +0800, Boqun Feng wrote:
>>
>> >>
>> >> __srcu_read_lock() used to be called with preemption disabled. I guess
>> >> the reason was because we have two percpu variables to increase. So with
>> >> only one percpu right, could we remove the preempt_{dis,en}able() in
>> >> srcu_read_lock() and use this_cpu_inc() here?
>> >
>> > Quite possibly...
>> >
>>
>
> Hello, Lai ;-)
>
>> it will be nicer if it is removed.
>>
>> The reason for the preemption-disabled was also because we
>> have to disallow any preemption between the fetching of the idx
>> and the increasement. so that we have at most NR_CPUS worth
>> of readers using the old index that haven't incremented the counters.
>>
>
> After reading the comment for a while, I actually got a question, maybe
> I miss something ;-)
>
> Why "at most NR_CPUS worth of readers using the old index haven't
> incremented the counters" could save us from overflow the counter?
>
> Please consider the following case in current implementation:
>
>
> {sp->completed = 0} so idx = 1 in srcu_advance_batches(...)
>
> one thread A is currently in __srcu_read_lock() and using idx = 1 and
> about to increase the percpu c[idx], and ULONG_MAX __srcu_read_lock()s
> have been called and returned with idx = 1, please note I think this is
> possible because I assume we may have some code like this:
>
>         unsigned long i = 0;
>         for (; i < ULONG_MAX; i++)
>                 srcu_read_lock(); // return the same idx 1;

this is the wrong usage of the api.


you might rewrite it as:

        unsigned long index[2] = {0, 0};
        unsigned long i = 0;
        for (;  index[1] < ULONG_MAX; i++)
                 index[srcu_read_lock()]++;


I think we should add document to disallow this kind of usage.
a reader should eat 4bytes on the memory at least.

>
> And none of the corresponding srcu_read_unlock() has been called;
>
> In this case, at the time thread A increases the percpu c[idx], that
> will result in an overflow, right? So even one reader using old idx will
> result in overflow.
>
>
> I think we won't be hit by overflow is not because we have few readers
> using old idx, it's because there are unlikely ULONG_MAX + 1
> __srcu_read_lock() called for the same idx, right? And the reason of
> this is much complex: because we won't have a fair mount of threads in
> the system, because no thread will nest srcu many levels, because there
> won't be a lot readers using old idx.
>
> And this will still be true if we use new mechanism and shrink the
> preemption disabled section, right?
>
> Regards,
> Boqun
>
>> if we remove the preempt_{dis,en}able(). we must change the
>> "NR_CPUS" in the comment into ULONG_MAX/4. (I assume
>> one on-going reader needs at least need 4bytes at the stack). it is still safe.
>>
>> but we still need to think more if we want to remove the preempt_{dis,en}able().
>>
>> Thanks
>> Lai