If rcu_print_task_stall() is invoked on an rcu_node structure that does not contain any tasks blocking the current grace period, it takes an early exit that fails to release that rcu_node structure's lock. This results in a self-deadlock, which is detected by lockdep. To reproduce this bug: tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --duration 3 --trust-make --configs "TREE03" --kconfig "CONFIG_PROVE_LOCKING=y" --bootargs "rcutorture.stall_cpu=30 rcutorture.stall_cpu_block=1 rcutorture.fwd_progress=0 rcutorture.test_boost=0" This will also result in other complaints, including RCU's scheduler hook complaining about blocking rather than preemption and an rcutorture writer stall. Only a partial RCU CPU stall warning message will be printed because of the self-deadlock. This commit therefore releases the lock on the rcu_print_task_stall() function's early exit path.
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| @@ -267,8 +267,10 @@ static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags) · 266 unmodified lines | ||
| 267 | 267 | struct task_struct *ts[8]; |
| 268 | 268 | |
| 269 | 269 | lockdep_assert_irqs_disabled(); |
| 270 | − if (!rcu_preempt_blocked_readers_cgp(rnp)) | |
| 270 | + if (!rcu_preempt_blocked_readers_cgp(rnp)) { | |
| 271 | + raw_spin_unlock_irqrestore_rcu_node(rnp, flags); | |
| 271 | 272 | return 0; |
| 273 | + } | |
| 272 | 274 | pr_err("\tTasks blocked on level-%d rcu_node (CPUs %d-%d):", |
| 273 | 275 | rnp->level, rnp->grplo, rnp->grphi); |
| 274 | 276 | t = list_entry(rnp->gp_tasks->prev, |