Re: [PATCH 01/20] asm-generic/mmiowb: Add generic implementation of mmiowb() tracking

[Michael Ellerman]

Nicholas Piggin <npiggin@xxxxxxxxx> writes:
> Michael Ellerman's on March 3, 2019 7:26 pm:
>> Nicholas Piggin <npiggin@xxxxxxxxx> writes:
...
>>> what was broken about the powerpc one, which is basically:
>>>
>>> static inline void mmiowb_set_pending(void)
>>> {
>>> 	struct mmiowb_state *ms = __mmiowb_state();
>>> 	ms->mmiowb_pending = 1;
>>> }
>>>
>>> static inline void mmiowb_spin_lock(void)
>>> {
>>> }
>> 
>> The current powerpc code clears io_sync in spin_lock().
>> 
>> ie, it would be equivalent to:
>> 
>> static inline void mmiowb_spin_lock(void)
>> {
>>  	ms->mmiowb_pending = 0;
>> }
>
> Ah okay that's what I missed. How about we just not do that?

Yeah I thought of that too but it's not great. We'd start semi-randomly
executing the sync in unlock depending on whether someone had done IO on
that CPU prior to the spinlock.

eg.

	writel(x, y);		// sets paca->io_sync
	...	

	<schedule>

	spin_lock(a);
        ...
        // No IO in here
        ...
        spin_unlock(a);		// sync() here because other task did writel().


Which wouldn't be *incorrect*, but would be kind of weird.

cheers