Messages in this thread Patch in this message | | | Date | Tue, 31 May 2016 12:03:15 +0100 | From | Mark Rutland <> | Subject | Re: [PATCHv3] rcu: tree: correctly handle sparse possible cpus |
| |
On Mon, May 23, 2016 at 09:13:33AM -0700, Paul E. McKenney wrote: > On Mon, May 23, 2016 at 11:42:59AM +0100, Mark Rutland wrote: > > In many cases in the RCU tree code, we iterate over the set of cpus for > > a leaf node described by rcu_node::grplo and rcu_node::grphi, checking > > per-cpu data for each cpu in this range. However, if the set of possible > > cpus is sparse, some cpus described in this range are not possible, and > > thus no per-cpu region will have been allocated (or initialised) for > > them by the generic percpu code. > > > > Erroneous accesses to a per-cpu area for these !possible cpus may fault > > or may hit other data depending on the addressed generated when the > > erroneous per cpu offset is applied. In practice, both cases have been > > observed on arm64 hardware (the former being silent, but detectable with > > additional patches). > > > > To avoid issues resulting from this, we must iterate over the set of > > *possible* cpus for a given leaf node. This patch add a new helper, > > for_each_leaf_node_possible_cpu, to enable this. As iteration is often > > intertwined with rcu_node local bitmask manipulation, a new > > leaf_node_cpu_bit helper is added to make this simpler and more > > consistent. The RCU tree code is made to use both of these where > > appropriate. > > Thank you, Mark, queued for review and further testing. > > Thanx, Paul
Thanks Paul.
Unfortunately, it seems that in my haste to spin v3, I missed your suggested logic to handle the !cpu_possible(rnp->grplo) case [1]. Sorry about that, evidently I was not being sufficiently thorough.
Would you be happy to fold that in, as per the diff below? Otherwise I can send that as a fixup patch, or a respin the whole thing as v4, per your preference.
I've given the below a spin on arm64 atop of -rcu/dev, with and without RCU_BOOST and/or RCU_TRACE, and I've build-tested likewise for x86. I've devised and tested the !cpu_possible(rnp->grplo) case by messing with the arm64 SMP code and the RCU tree fanout. So far everything seems happy: no build issues, UBSAN splats, or other runtime failures.
So fingers crossed, that's the last issue with this patch blatted...
Thanks, Mark.
[1] lkml.kernel.org/r/20160517190106.GJ3528@linux.vnet.ibm.com
---->8---- diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h index dc0b7da..f714f87 100644 --- a/kernel/rcu/tree.h +++ b/kernel/rcu/tree.h @@ -291,7 +291,7 @@ struct rcu_node { * Iterate over all possible CPUs in a leaf RCU node. */ #define for_each_leaf_node_possible_cpu(rnp, cpu) \ - for ((cpu) = rnp->grplo; \ + for ((cpu) = cpumask_next(rnp->grplo - 1, cpu_possible_mask); \ cpu <= rnp->grphi; \ cpu = cpumask_next((cpu), cpu_possible_mask))
| |