Messages in this thread |  | | | Date | Sun, 2 Mar 2008 23:28:35 +0100 | | From | "Dmitry Adamushko" <> | | Subject | Re: [PATCH 1/2] kthread: add a missing memory barrier to kthread_stop() |
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On 02/03/2008, Rusty Russell <rusty@rustcorp.com.au> wrote: > On Thursday 21 February 2008 08:36:30 Dmitry Adamushko wrote: > > From: Dmitry Adamushko <dmitry.adamushko@gmail.com> > > Subject: kthread: add a missing memory barrier to kthread_stop() > > > > We must ensure that kthread_stop_info.k has been updated before > > kthread's wakeup. This is required to properly support > > the use of kthread_should_stop() in the main loop of kthread. > > > > wake_up_process() doesn't imply a full memory barrier, > > so we add an explicit one. > > > I always believed that wake_up_process() implies a write barrier. It's pretty > common to set something up then wake the intended recipient.
No, it didn't imply a write mb. Moreover, the situation I tried to address required a full mb (write vs. read ops. synchronization).
Please take a look at the discussion here http://lkml.org/lkml/2008/2/23/238
and here is a more detailed description of the problem : http://groups.google.com/group/fa.linux.kernel/browse_thread/thread/44c45685680585fc/e58785df0eeee6f8?lnk=raot > > So I think this patch is overkill, but I'm happy to be corrected.
We ended up adding an explicit smp_wmb() (a patch by Linus) to the very beginning of try_to_wake_up(). Combined with the following spin_lock() it acts as a 'full' mb for write vs. read ('read' takes place inside try_to_wake_up()) and that's exactly what we need here.
The 1st link mentioned above may provide more details for interested readers.
So yes, now that we have got smp_wmb() in try_to_wake_up() this patch is redundant.
> > Thanks! > > Rusty. >
-- Best regards, Dmitry Adamushko
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