Re: [RFC PATCH] irqchip/gic-v3: Make gic_handle_irq() notrace

[Marc Zyngier]

Hi Zenghui,

On 29/03/2019 13:23, Zenghui Yu wrote:
> Enable pseudo NMI together with function_graph tracer, will lead
> the system to a hang. This is easy to reproduce,
> 
>   1) Set "irqchip.gicv3_pseudo_nmi=1" on the kernel command line
>   2) echo function_graph > /sys/kernel/debug/tracing/current_tracer
> 
> This patch (RFC) set gic_handle_irq() as notrace and it seems works
> fine now. But I have no idea about what the issue is exactly, and
> you can regard this patch as a report then :)
> 
> Can someone give a look at it and provide some explanations ?
> 
> Thanks!
> 
> Cc: Julien Thierry <julien.thierry@xxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Signed-off-by: Zenghui Yu <yuzenghui@xxxxxxxxxx>
> ---
>  drivers/irqchip/irq-gic-v3.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/irqchip/irq-gic-v3.c b/drivers/irqchip/irq-gic-v3.c
> index 15e55d3..8d0c25f 100644
> --- a/drivers/irqchip/irq-gic-v3.c
> +++ b/drivers/irqchip/irq-gic-v3.c
> @@ -487,7 +487,7 @@ static inline void gic_handle_nmi(u32 irqnr, struct pt_regs *regs)
>  		gic_deactivate_unhandled(irqnr);
>  }
>  
> -static asmlinkage void __exception_irq_entry gic_handle_irq(struct pt_regs *regs)
> +static asmlinkage notrace void __exception_irq_entry gic_handle_irq(struct pt_regs *regs)
>  {
>  	u32 irqnr;
>  
> 


That's interesting. Do you have any out of tree patch that actually
makes use of the pseudo-NMI feature? Without those patches, the
behaviour should stay unchanged.

On the other hand, if you can generate pseudo-NMIs, you could end-up
tracing gic_handle_irq whilst being inside the tracing code with
interrupts being notionally disabled (and that could be pretty bad).

So, patches or no patches?

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...