Messages in this thread | | | Date | Tue, 31 Oct 2023 13:18:21 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] sched: Don't call any kfree*() API in do_set_cpus_allowed() |
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On Tue, Oct 31, 2023 at 11:52:38AM +0100, Frederic Weisbecker wrote: > On Tue, Oct 31, 2023 at 09:53:08AM +0100, Peter Zijlstra wrote: > > On Mon, Oct 30, 2023 at 08:14:18PM -0400, Waiman Long wrote: > > > Commit 851a723e45d1 ("sched: Always clear user_cpus_ptr in > > > do_set_cpus_allowed()") added a kfree() call to free any user > > > provided affinity mask, if present. It was changed later to use > > > kfree_rcu() in commit 9a5418bc48ba ("sched/core: Use kfree_rcu() > > > in do_set_cpus_allowed()") to avoid a circular locking dependency > > > problem. > > > > > > It turns out that even kfree_rcu() isn't safe for avoiding > > > circular locking problem. As reported by kernel test robot, > > > the following circular locking dependency still exists: > > > > > > &rdp->nocb_lock --> rcu_node_0 --> &rq->__lock > > > > > > So no kfree*() API can be used in do_set_cpus_allowed(). To prevent > > > memory leakage, the unused user provided affinity mask is now saved in a > > > lockless list to be reused later by subsequent sched_setaffinity() calls. > > > > > > Without kfree_rcu(), the internal cpumask_rcuhead union can be removed > > > too as a lockless list entry only holds a single pointer. > > > > > > Fixes: 851a723e45d1 ("sched: Always clear user_cpus_ptr in do_set_cpus_allowed()") > > > > Bah, or we fix RCU... Paul, how insane is the below? > > Makes sense. We can't remove &rdp->nocb_lock --> rcu_node_0 but we can (and > should) indeed remove rcu_node_0 --> &rq->__lock > > Just a detail below: > > > @@ -2284,10 +2289,13 @@ static void force_qs_rnp(int (*f)(struct rcu_data *rdp)) > > } > > for_each_leaf_node_cpu_mask(rnp, cpu, rnp->qsmask) { > > rdp = per_cpu_ptr(&rcu_data, cpu); > > - if (f(rdp)) { > > + ret = f(rdp); > > + if (ret > 0) { > > mask |= rdp->grpmask; > > rcu_disable_urgency_upon_qs(rdp); > > } > > + if (ret < 0) > > + rsmask |= 1UL << (cpu - rnp->grplo); > > I guess this can be simplified with rsmask |= rdp->grpmask;
Ah, I wasn't sure that was actually the same. I looked for a minute and gave up :-)
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