Messages in this thread | | | Date | Thu, 17 Nov 2016 06:38:29 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RFC tip/core/rcu] SRCU rewrite |
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On Thu, Nov 17, 2016 at 05:49:57AM -0800, Paul E. McKenney 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@linux.vnet.ibm.com> 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.
And one area needing special thought is the call to __srcu_read_lock() and __srcu_read_unlock() in do_exit().
Thanx, Paul
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