Re: [PATCH v10 1/2] printk: Make printk() completely async

[Petr Mladek]

On Mon 2016-08-22 13:15:20, Sergey Senozhatsky wrote:
> Hello,
> 
> On (08/20/16 14:24), Sergey Senozhatsky wrote:
> > On (08/19/16 21:00), Jan Kara wrote:
> > > > > depending on .config BUG() may never return back -- passing control
> > > > > to do_exit(), so printk_deferred_exit() won't be executed. thus we
> > > > > probably need to have a per-cpu variable that would indicate that
> > > > > we are in deferred_bug. hm... but do we really need deferred BUG()
> > > > > in the first place?
> > > > 
> since we are basically interested in wake_up_process() only from
> printk() POV. not sure how acceptable 2 * preempt_count and 2 * per-CPU
> writes for every try_to_wake_up().
> 
> 
> the other thing I just thought of is doing something as follows
> !!!not tested, will not compile, just an idea!!!
> 
> ---
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 6e260a0..bb8d719 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1789,6 +1789,7 @@ asmlinkage int vprintk_emit(int facility, int level,
>         printk_delay();
>  
>         local_irq_save(flags);
> +       printk_nmi_enter();
>         this_cpu = smp_processor_id();
>  
>         /*
> @@ -1804,6 +1805,7 @@ asmlinkage int vprintk_emit(int facility, int level,
>                  */
>                 if (!oops_in_progress && !lockdep_recursing(current)) {
>                         recursion_bug = true;
> +                       printk_nmi_exit();
>                         local_irq_restore(flags);
>                         return 0;
>                 }
> @@ -1920,6 +1922,7 @@ asmlinkage int vprintk_emit(int facility, int level,
>         logbuf_cpu = UINT_MAX;
>         raw_spin_unlock(&logbuf_lock);
>         lockdep_on();
> +       printk_nmi_exit();
>         local_irq_restore(flags);
>  
>         /* If called from the scheduler, we can not call up(). */

I was so taken by the idea of temporary forcing a lockless and
"trivial" printk implementation that I missed one thing.

Your patch use the alternative printk() variant around logbuf_lock.
But this is not the problem with wake_up_process(). printk_deferred()
takes logbuf_lock without problems.

Our problem is with calling wake_up_process() recursively. The
deadlock is in the scheduler locks.

But the patch still inspired me. What about blocking the problematic
wake_up_process() call by a per-cpu variable. I mean something like
this completely untested code:

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index ca9733b802ce..93915eb1fd0d 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1708,6 +1708,8 @@ static size_t cont_print_text(char *text, size_t size)
        return textlen;
 }
 
+DEFINE_PER_CPU(bool, printk_wakeup);
+
 asmlinkage int vprintk_emit(int facility, int level,
                            const char *dict, size_t dictlen,
                            const char *fmt, va_list args)
@@ -1902,8 +1904,17 @@ asmlinkage int vprintk_emit(int facility, int level,
        lockdep_off();
 
        if  (printk_kthread && !in_panic) {
+               bool __percpu *printk_wakeup_ptr;
+
                /* Offload printing to a schedulable context. */
-               wake_up_process(printk_kthread);
+               local_irq_save(flags);
+               printk_wake_up_ptr = this_cpu_ptr(&printk_wake_up);
+               if (!*printk_wakeup_ptr) {
+                       *printk_wake_up_ptr = true;
+                       wake_up_process(printk_kthread);
+                       *printk_wake_up_ptr = false;
+               }
+               local_irq_restore(flags);
                goto out_lockdep;
        } else {
                /*


We might eventually hide this into a wake_up_process_safe() or so.

Also we might need to use it also in console_unlock() to avoid similar
recursion there as well.

Best Regards,
Petr