[PATCH] rcu: Add missing unlock in rcu_print_task_stall

[Paul E. McKenney]

On Tue, Jul 27, 2021 at 03:45:42PM +0800, Cheng Jui Wang wrote:
> We encouterd a deadlock with following lockdep warning. The
> rcu_print_task_stall is supposed to release rnp->lock, but may just
> return without unlock.
> 
> 	if (!rcu_preempt_blocked_readers_cgp(rnp))
> 		return 0;
> 
> Add missing unlock before return to fix it.
> 
> ============================================
> WARNING: possible recursive locking detected
> 5.10.43
> --------------------------------------------
> swapper/7/0 is trying to acquire lock:
> ffffffc01268c018 (rcu_node_0){-.-.}-{2:2}, at: rcu_dump_cpu_stacks+0x94/0x138
> 
> but task is already holding lock:
> ffffffc01268c018 (rcu_node_0){-.-.}-{2:2}, at: check_cpu_stall+0x34c/0x6f8
> 
> other info that might help us debug this:
>  Possible unsafe locking scenario:
> 
>        CPU0
>        ----
>   lock(rcu_node_0);
>   lock(rcu_node_0);
> 
>  *** DEADLOCK ***
> 
>  May be due to missing lock nesting notation
> 
> 1 lock held by swapper/7/0:
>  #0: ffffffc01268c018 (rcu_node_0){-.-.}-{2:2},  at: check_cpu_stall+0x34c/0x6f8
> 
> stack backtrace:
> CPU: 7 PID: 0 Comm: swapper/7
> Call trace:
>  dump_backtrace.cfi_jt+0x0/0x8
>  show_stack+0x1c/0x2c
>  dump_stack_lvl+0xd8/0x16c
>  validate_chain+0x2124/0x2d34
>  __lock_acquire+0x7e4/0xed4
>  lock_acquire+0x114/0x394
>  _raw_spin_lock_irqsave+0x88/0xd4
>  rcu_dump_cpu_stacks+0x94/0x138
>  check_cpu_stall+0x498/0x6f8
>  rcu_sched_clock_irq+0xd4/0x214
>  update_process_times+0xb4/0xf4
>  tick_sched_timer+0x98/0x110
>  __hrtimer_run_queues+0x19c/0x2bc
>  hrtimer_interrupt+0x10c/0x3a8
>  arch_timer_handler_phys+0x5c/0x98
>  handle_percpu_devid_irq+0xe0/0x2a8
>  __handle_domain_irq+0xd0/0x19c
>  gic_handle_irq+0x6c/0x134
>  el1_irq+0xe0/0x1c0
>  arch_cpu_idle+0x1c/0x30
>  default_idle_call+0x58/0xcc
>  do_idle.llvm.13807299673429836468+0x118/0x2e8
>  cpu_startup_entry+0x28/0x2c
>  secondary_start_kernel+0x1d0/0x23c
> 
> Signed-off-by: Cheng Jui Wang <cheng-jui.wang at mediatek.com>

Good catch, thank you!

However, Yanfei Xu beat you to this with commit f6b3995a8b56dc ("rcu:
Fix stall-warning deadlock due to non-release of rcu_node ->lock"),
which is in -rcu and slated for the upcoming merge window.

His commit 8baded711edc ("rcu: Fix to include first blocked task in
stall warning") might also be of interest to you.

							Thanx, Paul

> ---
>  kernel/rcu/tree_stall.h | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> index 6c76988cc019..3dc464d4d9a5 100644
> --- a/kernel/rcu/tree_stall.h
> +++ b/kernel/rcu/tree_stall.h
> @@ -267,8 +267,10 @@ static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags)
>  	struct task_struct *ts[8];
>  
>  	lockdep_assert_irqs_disabled();
> -	if (!rcu_preempt_blocked_readers_cgp(rnp))
> +	if (!rcu_preempt_blocked_readers_cgp(rnp)) {
> +		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
>  		return 0;
> +	}
>  	pr_err("\tTasks blocked on level-%d rcu_node (CPUs %d-%d):",
>  	       rnp->level, rnp->grplo, rnp->grphi);
>  	t = list_entry(rnp->gp_tasks->prev,
> -- 
> 2.18.0