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

[Boqun Feng]

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;

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

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