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

From: Paul E. McKenney
Date: Thu Nov 17 2016 - 13:45:33 EST

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...
> >
> 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.
> 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().

Good points! Agreed, any change in the preemption needs careful thought
and needs to be a separate patch.

Thanx, Paul