rcu: Don't invoke try_invoke_on_locked_down_task() with irqs disabled

The try_invoke_on_locked_down_task() function requires that
interrupts be enabled, but it is called with interrupts disabled from
rcu_print_task_stall(), resulting in an "IRQs not enabled as expected"
diagnostic.  This commit therefore updates rcu_print_task_stall()
to accumulate a list of the first few tasks while holding the current
leaf rcu_node structure's ->lock, then releases that lock and only then
uses try_invoke_on_locked_down_task() to attempt to obtain per-task
detailed information.  Of course, as soon as ->lock is released, the
task might exit, so the get_task_struct() function is used to prevent
the task structure from going away in the meantime.

Link: https://lore.kernel.org/lkml/000000000000903d5805ab908fc4@google.com/
Fixes: 5bef8da66a9c ("rcu: Add per-task state to RCU CPU stall warnings")
Reported-by: syzbot+cb3b69ae80afd6535b0e@syzkaller.appspotmail.com
Reported-by: syzbot+f04854e1c5c9e913cc27@syzkaller.appspotmail.com
Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index 0fde39b..ca21d28 100644 (file)
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -249,13 +249,16 @@ static bool check_slow_task(struct task_struct *t, void *arg)
 
 /*
  * Scan the current list of tasks blocked within RCU read-side critical
- * sections, printing out the tid of each.
+ * sections, printing out the tid of each of the first few of them.
  */
-static int rcu_print_task_stall(struct rcu_node *rnp)
+static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags)
+       __releases(rnp->lock)
 {
+       int i = 0;
        int ndetected = 0;
        struct rcu_stall_chk_rdr rscr;
        struct task_struct *t;
+       struct task_struct *ts[8];
 
        if (!rcu_preempt_blocked_readers_cgp(rnp))
                return 0;
@@ -264,6 +267,14 @@ static int rcu_print_task_stall(struct rcu_node *rnp)
        t = list_entry(rnp->gp_tasks->prev,
                       struct task_struct, rcu_node_entry);
        list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
+               get_task_struct(t);
+               ts[i++] = t;
+               if (i >= ARRAY_SIZE(ts))
+                       break;
+       }
+       raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+       for (i--; i; i--) {
+               t = ts[i];
                if (!try_invoke_on_locked_down_task(t, check_slow_task, &rscr))
                        pr_cont(" P%d", t->pid);
                else
@@ -273,6 +284,7 @@ static int rcu_print_task_stall(struct rcu_node *rnp)
                                ".q"[rscr.rs.b.need_qs],
                                ".e"[rscr.rs.b.exp_hint],
                                ".l"[rscr.on_blkd_list]);
+               put_task_struct(t);
                ndetected++;
        }
        pr_cont("\n");
@@ -293,8 +305,9 @@ static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
  * Because preemptible RCU does not exist, we never have to check for
  * tasks blocked within RCU read-side critical sections.
  */
-static int rcu_print_task_stall(struct rcu_node *rnp)
+static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags)
 {
+       raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
        return 0;
 }
 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
@@ -472,7 +485,6 @@ static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps)
        pr_err("INFO: %s detected stalls on CPUs/tasks:\n", rcu_state.name);
        rcu_for_each_leaf_node(rnp) {
                raw_spin_lock_irqsave_rcu_node(rnp, flags);
-               ndetected += rcu_print_task_stall(rnp);
                if (rnp->qsmask != 0) {
                        for_each_leaf_node_possible_cpu(rnp, cpu)
                                if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) {
@@ -480,7 +492,7 @@ static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps)
                                        ndetected++;
                                }
                }
-               raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+               ndetected += rcu_print_task_stall(rnp, flags); // Releases rnp->lock.
        }
 
        for_each_possible_cpu(cpu)