Re: [PATCH] blktrace: fix 'BUG: sleeping function called from invalid context' in case of PREEMPT_RT

[Steven Rostedt <rostedt@goodmis.org>]

On Mon, 14 Dec 2020 10:22:17 +0800
Ming Lei <ming.lei@redhat.com> wrote:

> trace_note_tsk() is called by __blk_add_trace(), which is covered by RCU read lock.
> So in case of PREEMPT_RT, warning of 'BUG: sleeping function called from invalid context'
> will be triggered because spin lock is converted to rtmutex.

The RCU read_lock() can not be the cause of this issue, because under
PREEMPT_RT, rcu_read_lock() can be preempted.

What was the full back trace of this problem?

> 
> Fix the issue by converting running_trace_lock into raw_spin_lock().
> 
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
>  kernel/trace/blktrace.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
> index 2c5b3c5317c2..53dc876d669d 100644
> --- a/kernel/trace/blktrace.c
> +++ b/kernel/trace/blktrace.c
> @@ -34,7 +34,7 @@ static struct trace_array *blk_tr;
>  static bool blk_tracer_enabled __read_mostly;
>  
>  static LIST_HEAD(running_trace_list);
> -static __cacheline_aligned_in_smp DEFINE_SPINLOCK(running_trace_lock);
> +static __cacheline_aligned_in_smp DEFINE_RAW_SPINLOCK(running_trace_lock);
>  
>  /* Select an alternative, minimalistic output than the original one */
>  #define TRACE_BLK_OPT_CLASSIC	0x1
> @@ -121,12 +121,12 @@ static void trace_note_tsk(struct task_struct *tsk)
>  	struct blk_trace *bt;
>  
>  	tsk->btrace_seq = blktrace_seq;
> -	spin_lock_irqsave(&running_trace_lock, flags);
> +	raw_spin_lock_irqsave(&running_trace_lock, flags);
>  	list_for_each_entry(bt, &running_trace_list, running_list) {
>  		trace_note(bt, tsk->pid, BLK_TN_PROCESS, tsk->comm,
>  			   sizeof(tsk->comm), 0);
>  	}

How big is this running_trace_list? May not be something we want raw locks
around.

Please understand that converting locks to raw should be the last resort.
One should always look at the reason for a spin lock in a preempt disabled
area and see if there's other means of solving it before simply switch a
lock to raw, as each raw spinlock makes PREEMPT_RT less real time.

-- Steve


> -	spin_unlock_irqrestore(&running_trace_lock, flags);
> +	raw_spin_unlock_irqrestore(&running_trace_lock, flags);
>  }
>  
>  static void trace_note_time(struct blk_trace *bt)
> @@ -669,9 +669,9 @@ static int __blk_trace_startstop(struct request_queue *q, int start)
>  			blktrace_seq++;
>  			smp_mb();
>  			bt->trace_state = Blktrace_running;
> -			spin_lock_irq(&running_trace_lock);
> +			raw_spin_lock_irq(&running_trace_lock);
>  			list_add(&bt->running_list, &running_trace_list);
> -			spin_unlock_irq(&running_trace_lock);
> +			raw_spin_unlock_irq(&running_trace_lock);
>  
>  			trace_note_time(bt);
>  			ret = 0;
> @@ -679,9 +679,9 @@ static int __blk_trace_startstop(struct request_queue *q, int start)
>  	} else {
>  		if (bt->trace_state == Blktrace_running) {
>  			bt->trace_state = Blktrace_stopped;
> -			spin_lock_irq(&running_trace_lock);
> +			raw_spin_lock_irq(&running_trace_lock);
>  			list_del_init(&bt->running_list);
> -			spin_unlock_irq(&running_trace_lock);
> +			raw_spin_unlock_irq(&running_trace_lock);
>  			relay_flush(bt->rchan);
>  			ret = 0;
>  		}