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
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