RE: spinlock deadlock

[buyitian <buyit@xxxxxxx>]

 

> From: mulyadi.santosa@xxxxxxxxx
> Date: Sun, 17 Feb 2013 12:37:21 +0700
> Subject: Re: spinlock deadlock
> To: buyit@xxxxxxx
> CC: kernelnewbies@xxxxxxxxxxxxxxxxx
>
> On Fri, Feb 15, 2013 at 3:52 PM, buyitian <buyit@xxxxxxx> wrote:
> > is it possible that printk cause deadlock? the path is as below:
> >
> > 1. taskA runs on CPU0, and run schedule to acqire the rq->lock.
> > 2. taskA calls printk while holding rq->lock.
>
> I began to get rusty on this...but, why do you grab run queue lock?
>
>

 

   i want to know why the authors use spinlock and semaphore like this, because the change history is very insteresting:

1. in patch 0b5e1c5255e7ee8670e077e8224e5c2281229a5b: Peter did below changes:

    (1). in function console_trylock_for_printk():

if (!can_use_console(cpu)) {
    console_locked = 0;
-   up(&console_sem);
+   wake = 1;
    retval = 0;
   }
  }
  printk_cpu = UINT_MAX;
  spin_unlock(&logbuf_lock);
+ if (wake)
+  up(&console_sem);

 

          (2). in function console_unlock():

 

- up(&console_sem);
  spin_unlock_irqrestore(&logbuf_lock, flags);
+ up(&console_sem);
  if (wake_klogd)
   wake_up_klogd();
 } 

 

         this patch is to prevent deadlock between rq->lock and logbuf_lock. i can understand this thanks to Rabin.

 

2. in patch 07354eb1a74d1e1ece29f8bafe0b46e8c77a95ef, Thomas did changes as below, which reverted the change from peter, i don't know why:

    in function console_trylock_for_printk():

 

- spin_unlock(&logbuf_lock);
  if (wake)
   up(&console_sem);
+ raw_spin_unlock(&logbuf_lock);

 

       and this change exists in the latest kernel source code. it seems that deadlock bewteen rq->lock and logbuf_lock comes back, who can explain this, thanks.

 

     BTW: if i place printk inside schedule(), i may get rq->lock before calling printk.

> --
> regards,
>
> Mulyadi Santosa
> Freelance Linux trainer and consultant
>
> blog: the-hydra.blogspot.com
> training: mulyaditraining.blogspot.com

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