On 11.07.2013, at 04:48, tiejun.chen wrote:
> On 07/10/2013 05:49 PM, Alexander Graf wrote:
>>
>> On 10.07.2013, at 08:02, Tiejun Chen wrote:
>>
>>> We should ensure the preemption cannot occur while calling get_paca()
>>> insdide hard_irq_disable(), otherwise the paca_struct may be the
>>> wrong one just after. And btw, we may update timing stats in this case.
>>>
>>> Signed-off-by: Tiejun Chen <tiejun.chen@xxxxxxxxxxxxx>
>>> ---
>>> arch/powerpc/kvm/booke.c | 2 ++
>>> 1 file changed, 2 insertions(+)
>>>
>>> diff --git a/arch/powerpc/kvm/booke.c b/arch/powerpc/kvm/booke.c
>>> index dcc94f0..9dae25d 100644
>>> --- a/arch/powerpc/kvm/booke.c
>>> +++ b/arch/powerpc/kvm/booke.c
>>> @@ -839,6 +839,7 @@ int kvmppc_handle_exit(struct kvm_run *run, struct kvm_vcpu *vcpu,
>>> WARN_ON(local_paca->irq_happened != 0);
>>> #endif
>>>
>>> + preempt_disable();
>>> /*
>>> * We enter with interrupts disabled in hardware, but
>>> * we need to call hard_irq_disable anyway to ensure that
>>> @@ -848,6 +849,7 @@ int kvmppc_handle_exit(struct kvm_run *run, struct kvm_vcpu *vcpu,
>>>
>>> /* update before a new last_exit_type is rewritten */
>>> kvmppc_update_timing_stats(vcpu);
>>> + preempt_enable();
>>
>> All of the code here is already called with interrupts disabled. I don't see how we could preempt then?
>
> But the kernel still check this in preempt case,
>
> #define get_paca() ((void) debug_smp_processor_id(), local_paca)
>
> then trigger that known call trace as I observed :)
>
> BUG: using smp_processor_id() in preemptible [00000000] code: qemu-system-ppc/2065
# define preemptible() (preempt_count() == 0 && !irqs_disabled())
So we are only hitting this BUG() when either preempt_count is 0 (which your patch is trying to fix) and at the same time interrupts are enabled. But wait - interrupts are disabled, aren't they? Let's check.
#define irqs_disabled() \
({ \
unsigned long _flags; \
raw_local_save_flags(_flags); \
raw_irqs_disabled_flags(_flags); \
})
#define raw_irqs_disabled_flags(flags) \
({ \
typecheck(unsigned long, flags); \
arch_irqs_disabled_flags(flags); \
})
static inline unsigned long arch_local_save_flags(void)
{
unsigned long flags;
asm volatile(
"lbz %0,%1(13)"
: "=r" (flags)
: "i" (offsetof(struct paca_struct, soft_enabled)));
return flags;
}
static inline bool arch_irqs_disabled_flags(unsigned long flags)
{
return flags == 0;
}
So we're running with soft_enabled == 0 here which means that irqs_disabled() also returns 0.
Ben, is soft_enabled == 0; hard_enabled == 1 a valid combination that may ever occur?
Alex
> caller is .kvmppc_handle_exit+0x48/0x810
> CPU: 0 PID: 2065 Comm: qemu-system-ppc Not tainted 3.10.0-172036-ge2daa28-dirty #116
> Call Trace:
> [c0000001fc637570] [c00000000000835c] .show_stack+0x7c/0x1f0 (unreliable)
> [c0000001fc637640] [c000000000673a0c] .dump_stack+0x28/0x3c
> [c0000001fc6376b0] [c0000000002f04d8] .debug_smp_processor_id+0x108/0x120
> [c0000001fc637740] [c0000000000444e8] .kvmppc_handle_exit+0x48/0x810
> [c0000001fc6377f0] [c000000000047c80] .kvmppc_resume_host+0xa4/0xf8
>
> Tiejun
>
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