[lkml]   [2012]   [Dec]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context
On 12/12/2012 11:53 PM, Oleg Nesterov wrote:
> On 12/12, Srivatsa S. Bhat wrote:
>> On 12/12/2012 10:54 PM, Oleg Nesterov wrote:
>>> And when I look at get_online_cpus_atomic() again it uses rmb(). This
>>> doesn't look correct, we need the full barrier between this_cpu_inc()
>>> and writer_active().
>> Hmm..
>>> At the same time reader_nested_percpu() can be checked before mb().
>> I thought that since the increment and the check (reader_nested_percpu)
>> act on the same memory location, they will naturally be run in the given
>> order, without any need for barriers. Am I wrong?
> And this is what I meant, you do not need a barrier before
> reader_nested_percpu().

Ah, ok!

> But you need to ensure that WRITE(reader_percpu_refcnt) and READ(writer_signal)
> can't be reordered, so you need mb() in between. rmb() can serialize LOADs and

OK, got it. (I know you meant s/can/can't).

I'm trying to see if we can somehow exploit the fact that the writer can
potentially tolerate if a reader ignores his signal (to switch to rwlocks)
for a while... and use this to get rid of barriers in the reader path (without
using synchronize_sched() at the writer, of course). And perhaps also take advantage
of the fact that the read_lock() acts as a one-way barrier..

I don't know, maybe its not possible after all.. :-/

Srivatsa S. Bhat

 \ /
  Last update: 2012-12-12 20:01    [W:0.061 / U:0.520 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site