Re: [PATCH] MIPS: Change definition of cpu_relax() for Loongson-3

[Huacai Chen]

On Wed, Jul 18, 2018 at 1:52 AM, Paul Burton <paul.burton@xxxxxxxx> wrote:
> Hi Huacai,
>
> On Fri, Jul 13, 2018 at 03:37:57PM +0800, Huacai Chen wrote:
>> Linux expects that if a CPU modifies a memory location, then that
>> modification will eventually become visible to other CPUs in the system.
>>
>> On Loongson-3 processor with SFB (Store Fill Buffer), loads may be
>> prioritised over stores so it is possible for a store operation to be
>> postponed if a polling loop immediately follows it. If the variable
>> being polled indirectly depends on the outstanding store [for example,
>> another CPU may be polling the variable that is pending modification]
>> then there is the potential for deadlock if interrupts are disabled.
>> This deadlock occurs in qspinlock code.
>>
>> This patch changes the definition of cpu_relax() to smp_mb() for
>> Loongson-3, forcing a flushing of the SFB on SMP systems before the
>> next load takes place. If the Kernel is not compiled for SMP support,
>> this will expand to a barrier() as before.
>>
>> References: 534be1d5a2da940 (ARM: 6194/1: change definition of cpu_relax() for ARM11MPCore)
>> Cc: stable@xxxxxxxxxxxxxxx
>> Signed-off-by: Huacai Chen <chenhc@xxxxxxxxxx>
>> ---
>>  arch/mips/include/asm/processor.h | 10 ++++++++++
>>  1 file changed, 10 insertions(+)
>>
>> diff --git a/arch/mips/include/asm/processor.h b/arch/mips/include/asm/processor.h
>> index af34afb..a8c4a3a 100644
>> --- a/arch/mips/include/asm/processor.h
>> +++ b/arch/mips/include/asm/processor.h
>> @@ -386,7 +386,17 @@ unsigned long get_wchan(struct task_struct *p);
>>  #define KSTK_ESP(tsk) (task_pt_regs(tsk)->regs[29])
>>  #define KSTK_STATUS(tsk) (task_pt_regs(tsk)->cp0_status)
>>
>> +#ifdef CONFIG_CPU_LOONGSON3
>> +/*
>> + * Loongson-3's SFB (Store-Fill-Buffer) may get starved when stuck in a read
>> + * loop. Since spin loops of any kind should have a cpu_relax() in them, force
>> + * a Store-Fill-Buffer flush from cpu_relax() such that any pending writes will
>> + * become available as expected.
>> + */
>
> I think "may starve writes" or "may queue writes indefinitely" would be
> clearer than "may get starved".
Need I change the comment and resend? Or you change the comment and get merged?

Huacai

>
>> +#define cpu_relax()  smp_mb()
>> +#else
>>  #define cpu_relax()  barrier()
>> +#endif
>>
>>  /*
>>   * Return_address is a replacement for __builtin_return_address(count)
>> --
>> 2.7.0
>
> Apart from the comment above though this looks better to me.
>
> Re-copying the LKMM maintainers - are you happy(ish) with this?
>
> Thanks,
>     Paul