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Date: Wed, 23 Dec 2015 13:15:46 +0900
From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Jan Kara <jack@suse.cz>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Petr Mladek <pmladek@suse.cz>, KY Sri nivasan <kys@microsoft.com>,
        Steven Rostedt <rostedt@goodmis.org>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/7] printk: Hand over printing to console if printing
 too long
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On (12/23/15 12:57), Sergey Senozhatsky wrote:
[..]
> > > 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++
> 
> 					or we huge enough number of CPUs, `deep' stack
> 					traces, slow serial and CPU doing dump_stack()
> 					under raw_spin_lock(&stop_lock), so it can take
> 					longer than 1 second to print the stacks and
> 					thus panic CPU will set oops_in_progress back
> 					to 0.
> 
> > >  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.

well, if panic_timeout != 0, then wake_up_klogd() calls irq_work_queue() which
schedule_work. what if we have the following

CPU0						CPU1 - CPUN

foo
preempt_disable
	bar
		panic				irq/fiq disable
			schedule_work		while (1) cpu_relax

	-ss