From: Paul E. McKenney Date: Tue, 11 Sep 2018 15:57:48 +0000 (-0700) Subject: rcu: Stop expedited grace periods from relying on stop-machine X-Git-Tag: v5.0-rc1~152^2^2~17^4~13 X-Git-Url: https://asedeno.scripts.mit.edu/gitweb/?a=commitdiff_plain;h=9cac83a57e99e4692315b7a91a81bab787961d97;p=linux.git rcu: Stop expedited grace periods from relying on stop-machine The CPU-selection code in sync_rcu_exp_select_cpus() disables preemption to prevent the cpu_online_mask from changing. However, this relies on the stop-machine mechanism in the CPU-hotplug offline code, which is not desirable (it would be good to someday remove the stop-machine mechanism). This commit therefore instead uses the relevant leaf rcu_node structure's ->ffmask, which has a bit set for all CPUs that are fully functional. A given CPU's bit is cleared very early during offline processing by rcutree_offline_cpu() and set very late during online processing by rcutree_online_cpu(). Therefore, if a CPU's bit is set in this mask, and preemption is disabled, we have to be before the synchronize_sched() in the CPU-hotplug offline code, which means that the CPU is guaranteed to be workqueue-ready throughout the duration of the enclosing preempt_disable() region of code. This also has the side-effect of using WORK_CPU_UNBOUND if all the CPUs for this leaf rcu_node structure are offline, which is an acceptable difference in behavior. Reported-by: Sebastian Andrzej Siewior Signed-off-by: Paul E. McKenney --- diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h index 8d18c1014e2b..e669ccf3751b 100644 --- a/kernel/rcu/tree_exp.h +++ b/kernel/rcu/tree_exp.h @@ -450,10 +450,12 @@ static void sync_rcu_exp_select_cpus(smp_call_func_t func) } INIT_WORK(&rnp->rew.rew_work, sync_rcu_exp_select_node_cpus); preempt_disable(); - cpu = cpumask_next(rnp->grplo - 1, cpu_online_mask); + cpu = find_next_bit(&rnp->ffmask, BITS_PER_LONG, -1); /* If all offline, queue the work on an unbound CPU. */ - if (unlikely(cpu > rnp->grphi)) + if (unlikely(cpu > rnp->grphi - rnp->grplo)) cpu = WORK_CPU_UNBOUND; + else + cpu += rnp->grplo; queue_work_on(cpu, rcu_par_gp_wq, &rnp->rew.rew_work); preempt_enable(); rnp->exp_need_flush = true;