Messages in this thread | | | Date | Mon, 4 Dec 2006 08:43:53 -0800 | From | "Paul E. McKenney" <> | Subject | Re: PATCH? rcu_do_batch: fix a pure theoretical memory ordering race |
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On Sun, Dec 03, 2006 at 12:25:17AM +0300, Oleg Nesterov wrote: > On top of rcu-add-a-prefetch-in-rcu_do_batch.patch > > rcu_do_batch: > > struct rcu_head *next, *list; > > while (list) { > next = list->next; <------ [1] > list->func(list); > list = next; > } > > We can't trust *list after list->func() call, that is why we load list->next > beforehand. However I suspect in theory this is not enough, suppose that > > - [1] is stalled > > - list->func() marks *list as unused in some way > > - another CPU re-uses this rcu_head and dirties it
The memory allocators are required to do whatever is required to ensure that the first CPU is no longer accessing the memory before allocating it to the second CPU.
So we should not need to do this -- and if we did, we would have serious problems throughout the kernel.
Thanx, Paul
> - [1] completes and gets a wrong result > > This means we need a barrier in between. mb() looks more suitable, but I think > rmb() should suffice. > > Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru> > > --- 19-rc6/kernel/rcupdate.c~rdp 2006-12-02 20:46:03.000000000 +0300 > +++ 19-rc6/kernel/rcupdate.c 2006-12-02 21:04:12.000000000 +0300 > @@ -236,6 +236,8 @@ static void rcu_do_batch(struct rcu_data > list = rdp->donelist; > while (list) { > next = list->next; > + /* complete the load above before we call ->func() */ > + smp_rmb(); > prefetch(next); > list->func(list); > list = next; > - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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