Re: [PATCH 1/7] printk: Hand over printing to console if printing too long

[Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>]

On (12/23/15 10:54), Sergey Senozhatsky wrote:
> On (12/22/15 14:47), Jan Kara wrote:
> [..]
> > @@ -1803,10 +1869,24 @@ asmlinkage int vprintk_emit(int facility, int level,
> >  	logbuf_cpu = UINT_MAX;
> >  	raw_spin_unlock(&logbuf_lock);
> >  	lockdep_on();
> > +	/*
> > +	 * By default we print message to console asynchronously so that kernel
> > +	 * doesn't get stalled due to slow serial console. That can lead to
> > +	 * softlockups, lost interrupts, or userspace timing out under heavy
> > +	 * printing load.
> > +	 *
> > +	 * However we resort to synchronous printing of messages during early
> > +	 * boot, when oops is in progress, or when synchronous printing was
> > +	 * explicitely requested by kernel parameter.
> > +	 */
> > +	if (keventd_up() && !oops_in_progress && !sync_print) {
> > +		__this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
> > +		irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
> > +	} else
> > +		sync_print = true;

oops, didn't have enough coffee... missed that `else sync_print = true' :(

	-ss

> >  	local_irq_restore(flags);
> 
> can we replace this oops_in_progress check with something more reliable?
> 
> CPU0                                CPU1 - CPUN
> panic()
>  local_irq_disable()                executing foo() with irqs disabled,
>  console_verbose()                  or processing an extremely long irq handler.
>  bust_spinlocks()
>     oops_in_progress++
> 
>  smp_send_stop()
> 
>  bust_spinlocks()
>     oops_in_progress--              ok, IPI arrives
>                                     dump_stack()/printk()/etc from IPI_CPU_STOP
> 			            "while (1) cpu_relax()" with irq/fiq disabled/halt/etc.
> 
> smp_send_stop() wrapped in `oops_in_progress++/oops_in_progress--' is arch specific,
> and some platforms don't do any IPI-delivered (e.g. via num_online_cpus()) checks at
> all. Some do. For example, arm/arm64:
> 
> void smp_send_stop(void)
> ...
>         /* Wait up to one second for other CPUs to stop */
>         timeout = USEC_PER_SEC;
>         while (num_online_cpus() > 1 && timeout--)
>                 udelay(1);
> 
>         if (num_online_cpus() > 1)
>                 pr_warn("SMP: failed to stop secondary CPUs\n");
> ...
> 
> 
> so there are non-zero chances that IPI will arrive to CPU after 'oops_in_progress--',
> and thus dump_stack()/etc. happening on that/those cpu/cpus will be lost.
> 
> 
> bust_spinlocks(0) does
> ...
> 	if (--oops_in_progress == 0)
> 		wake_up_klogd();
> ...
> 
> but local cpu has irqs disabled and `panic_timeout' can be zero.
> 
> How about setting 'sync_print' to 'true' in...
>   bust_spinlocks() /* only set to true */
> or
>   console_verbose() /* um... may be... */
> or
>   having a separate one-liner for that
> 
> void console_panic_mode(void)
> {
> 	sync_print = true;
> }
> 
> and call it early in panic(), before we send out IPI_STOP.
> 
> 	-ss
>