rcu: Use *_ONCE() to protect lockless ->expmask accesses

The rcu_node structure's ->expmask field is accessed locklessly when
starting a new expedited grace period and when reporting an expedited
RCU CPU stall warning.  This commit therefore handles the former by
taking a snapshot of ->expmask while the lock is held and the latter
by applying READ_ONCE() to lockless reads and WRITE_ONCE() to the
corresponding updates.

Link: https://lore.kernel.org/lkml/CANpmjNNmSOagbTpffHr4=Yedckx9Rm2NuGqC9UqE+AOz5f1-ZQ@mail.gmail.com
Reported-by: syzbot+134336b86f728d6e55a0@syzkaller.appspotmail.com
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Marco Elver <elver@google.com>

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index d632cd0..69c5aa6 100644 (file)
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -134,7 +134,7 @@ static void __maybe_unused sync_exp_reset_tree(void)
        rcu_for_each_node_breadth_first(rnp) {
                raw_spin_lock_irqsave_rcu_node(rnp, flags);
                WARN_ON_ONCE(rnp->expmask);
-               rnp->expmask = rnp->expmaskinit;
+               WRITE_ONCE(rnp->expmask, rnp->expmaskinit);
                raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
        }
 }
@@ -211,7 +211,7 @@ static void __rcu_report_exp_rnp(struct rcu_node *rnp,
                rnp = rnp->parent;
                raw_spin_lock_rcu_node(rnp); /* irqs already disabled */
                WARN_ON_ONCE(!(rnp->expmask & mask));
-               rnp->expmask &= ~mask;
+               WRITE_ONCE(rnp->expmask, rnp->expmask & ~mask);
        }
 }
 
@@ -241,7 +241,7 @@ static void rcu_report_exp_cpu_mult(struct rcu_node *rnp,
                raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
                return;
        }
-       rnp->expmask &= ~mask;
+       WRITE_ONCE(rnp->expmask, rnp->expmask & ~mask);
        __rcu_report_exp_rnp(rnp, wake, flags); /* Releases rnp->lock. */
 }
 
@@ -372,12 +372,10 @@ static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
        raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 
        /* IPI the remaining CPUs for expedited quiescent state. */
-       for_each_leaf_node_cpu_mask(rnp, cpu, rnp->expmask) {
+       for_each_leaf_node_cpu_mask(rnp, cpu, mask_ofl_ipi) {
                unsigned long mask = leaf_node_cpu_bit(rnp, cpu);
                struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 
-               if (!(mask_ofl_ipi & mask))
-                       continue;
 retry_ipi:
                if (rcu_dynticks_in_eqs_since(rdp, rdp->exp_dynticks_snap)) {
                        mask_ofl_test |= mask;
@@ -491,7 +489,7 @@ static void synchronize_sched_expedited_wait(void)
                                struct rcu_data *rdp;
 
                                mask = leaf_node_cpu_bit(rnp, cpu);
-                               if (!(rnp->expmask & mask))
+                               if (!(READ_ONCE(rnp->expmask) & mask))
                                        continue;
                                ndetected++;
                                rdp = per_cpu_ptr(&rcu_data, cpu);
@@ -503,7 +501,8 @@ static void synchronize_sched_expedited_wait(void)
                }
                pr_cont(" } %lu jiffies s: %lu root: %#lx/%c\n",
                        jiffies - jiffies_start, rcu_state.expedited_sequence,
-                       rnp_root->expmask, ".T"[!!rnp_root->exp_tasks]);
+                       READ_ONCE(rnp_root->expmask),
+                       ".T"[!!rnp_root->exp_tasks]);
                if (ndetected) {
                        pr_err("blocking rcu_node structures:");
                        rcu_for_each_node_breadth_first(rnp) {
@@ -513,7 +512,7 @@ static void synchronize_sched_expedited_wait(void)
                                        continue;
                                pr_cont(" l=%u:%d-%d:%#lx/%c",
                                        rnp->level, rnp->grplo, rnp->grphi,
-                                       rnp->expmask,
+                                       READ_ONCE(rnp->expmask),
                                        ".T"[!!rnp->exp_tasks]);
                        }
                        pr_cont("\n");
@@ -521,7 +520,7 @@ static void synchronize_sched_expedited_wait(void)
                rcu_for_each_leaf_node(rnp) {
                        for_each_leaf_node_possible_cpu(rnp, cpu) {
                                mask = leaf_node_cpu_bit(rnp, cpu);
-                               if (!(rnp->expmask & mask))
+                               if (!(READ_ONCE(rnp->expmask) & mask))
                                        continue;
                                dump_cpu_task(cpu);
                        }