Re: [RFC PATCH] docs/memory-barriers.txt: Rewrite "KERNEL I/O BARRIER EFFECTS" section

[Michael Ellerman]

Will Deacon <will.deacon@xxxxxxx> writes:
> [+more ppc folks]
>
> On Mon, Feb 18, 2019 at 04:50:12PM +0000, Will Deacon wrote:
>> On Wed, Feb 13, 2019 at 10:27:09AM -0800, Linus Torvalds wrote:
>> > Note that even if mmiowb() is expensive (and I don't think that's
>> > actually even the case on ia64), you can - and probably should - do
>> > what PowerPC does.
>> > 
>> > Doing an IO barrier on PowerPC is insanely expensive, but they solve
>> > that simply track the whole "have I done any IO" manually. It's not
>> > even that expensive, it just uses a percpu flag.
>> > 
>> > (Admittedly, PowerPC makes it less obvious that it's a percpu variable
>> > because it's actually in the special "paca" region that is like a
>> > hyper-local percpu area).
>
> [...]
>
>> > But we *could* first just do the mmiowb() unconditionally in the ia64
>> > unlocking code, and then see if anybody notices?
>> 
>> I'll hack this up as a starting point. We can always try to be clever later
>> on if it's deemed necessary.
>
> Ok, so I started hacking this up in core code with the percpu flag (since
> riscv apparently needs it), but I've now realised that I don't understand
> how the PowerPC trick works after all. Consider the following:
>
> 	spin_lock(&foo);	// io_sync = 0
> 	outb(42, port);		// io_sync = 1
> 	spin_lock(&bar);	// io_sync = 0
> 	...
> 	spin_unlock(&bar);
> 	spin_unlock(&foo);
>
> The inner lock could even happen in an irq afaict, but we'll end up skipping
> the mmiowb()/sync because the io_sync flag is unconditionally cleared by
> spin_lock(). Fixing this is complicated by the fact that I/O writes can be
> performed in preemptible context with no locks held, so we can end up
> spuriously setting the io_sync flag for arbitrary CPUs, hence the desire
> to clear it in spin_lock().
>
> If the paca entry was more than a byte, we could probably track that a
> spinlock is held and then avoid clearing the flag prematurely, but I have
> a feeling that I'm missing something. Anybody know how this is supposed to
> work?

I don't think you're missing anything :/

Having two flags like you suggest could work. Or you could just make the
flag into a nesting counter.

Or do you just remove the clearing from spin_lock()? 

That gets you:

 	spin_lock(&foo);
 	outb(42, port);		// io_sync = 1
 	spin_lock(&bar);
 	...
 	spin_unlock(&bar);	// mb(); io_sync = 0
 	spin_unlock(&foo);


And I/O outside of the lock case:

 	outb(42, port);		// io_sync = 1

 	spin_lock(&bar);
 	...
 	spin_unlock(&bar);	// mb(); io_sync = 0


Extra barriers are not ideal, but the odd spurious mb() might be
preferable to doing another compare and branch or increment in every
spin_lock()?

cheers